I'd not like to argue mate. Ships like obsidian as damage dealer is perfect for this mission. Single target ships or tbird have no chance of completing their spot. You say that outside units can come if you don't position yourself , but they have big aggression range so if u get into trouble you cannot run anywhere from spot. I don't use obsidian to lure all 4 units on me from mission. I take what i can kill instant and with 1 Repair Droid while baiters i leave always for after. Now i'll show you screenshot of every spot because i mastered sm since 1st try , but my mates can't simply complete all waves because of those ''outside mission units".
Orange area - 11/35 . It's small area with energy field and it's safe. If "Big Orbital Strike" fall there , you can either die , or run outside that spot where DS and marauders are. So it's ok spot in general , but if u take distance from enemies, etc. Horus you will lure marauder on you. So it's first reason to use graviton if you get caught , but mostly not necessary if not big trouble that time.
Yellow area - 25/66. It's area where ammons and rocketeers attack shelters at left side. If you stand even little on that side , Defenders come above wall. So , even if you don't stand near shelters , ammon and rocketeer will shoot it. You can always be on other side of spot which doesn't lure mantis on you , but your shelters (buildings) will suffer.
Light green area - 32/75. This area is completely free of defender range and provides spot where you can freely shoot. This spot is good.
Light blue area - 58/65. This spot has energy field behind where Custodian commander passes. However , shelters aren't in range of custodian. But on other side , Baiters constantly come and shoot shelters since it is their spot ( lvl 90 Baiters). This spot isn't available without Outside cover. So gravitons in use here are necessary if you are alone. This is one of hard spots for solo.
White area (above light blue) - 57/53. This area provides energy field where "Big Orbital Strike" can fall sometimes. Shelters are being split into that e field and area just above it. So , marauders from above can come and shoot that spot surely. If you go little under hill , there is chance of endurer coming ( not always ). If you go little right , Ammon lvl 92 , Baiters lvl 90 can also attack you as well. So you basically must stand at shelters and wait Horus to hit you if those things are near walls. Spot is ok and can be done well , if you can catch Marauders from up hill ( if you have Rockets , Sniper Blaster only). If you have Humpback etc. , no chance you can defend upper hill from marauders if they're not very near wall.
Purple area - 62/33. This is area on center of top hill where old Earth final missions are taking place. Area is filled with Signors and repogrubs. So you basically have to clean Signors while repogrubs are constantly there to be shot to not heal signors. MENDER constantly passes and heals them. So you have to kill mender , deal with signors who are covered by repogrubs while you must kill Mission units themselves. Spot is very bad , so even with gravitons use , you will fail , no matter how much you use.
Black area - 62/21. This area is isolated from any outside mantis , and rocket turrets cannot reach it. In this area you have space to run back to avoid Mission units range and keep shooting. But , if you don't have Membership , you must use AB ahead to refill , while Signors can make that refill even delayed and Mission units will come down eventually. Area is fine and i don't have problem with this area.
Red area - 88/38. This area is small and it's end of road. Behind you there area Signors , so no place to run back. If you stand at start of area , strikers from down spot can spot you and if Minelayer down lays any Mantis Mine near wall , it can affect you. At other side , if you even move there , Defenders can spot you and kill you surely because you can't run. In fact , you have to stand right at shelters up to wall which you Cannot do if Horus do their Bombs , if Baiters put their magnet trap , if Skirmishers and Strikers do Rocket and Chains. I would like to tell me , for Red and Yellow area , where do we need to stand to not get spotted and not die?
White (under red) - 96/49. This area has constant flow of Baiters who come and shoot shelters surely. Repogrubs are very good coming in there , while ressurectors know to interfere ( or even 2 ressurectors ). Defenders can come above wall even if you're not too near it , because they have High agression range. They often come even when you travel. This spot has no chance without Gravitons.
So , now tell me. What would you do for Yellow , Purple , Red and White ( under red ) Areas? I don't really need someone to teach me how to do missions because i mastered every trick and i have top items from planet.
Thunderbird - cannot use chain at all. No Gravitons - more mission units gather because of Baiters because untill you kill Baiter , other Mission units land on shelters. Outside units will just increase use of gravitons while you have to wait for coldown of them.
Obsidian - Best ship for mission. It can devastate units ( 3 units at once even ). It can survive and get enough coldowns for doing units. But still Baiters make problem since new Mission units land when you get coldowns after killing those. If outside units interfere , you can't defend without of use gravitons.
Emperor - This ship is mostly used by many players. They cannot complete any spot without use of gravitons. Outside units will make it immposible for completion.
Humpback - Ship can manage to do spots with gravitons , without them it can't manage to kill all in time , but it can survive. Outside units can make it fail again.
Goal to do mission is to not shoot mission units all at once , but to let 1 or 2 shoot shelters if you can't manage to pick 3 at once. All depends on what units land at shelter. If you don't have anywhere to move , you can survive and kill all mission units to some point , but your shelters will just get heavily damaged in end and be destroyed. Membership is recommended , because you don't have time to refill mostly ( only if you are that big Collector of BPs to go and complete all of them to try refill by some BPs that drop from units. Outside Cover is crucial for this mission , because Safest areas will not all appear at once. Even if they all appear , they are either too far , which will make shelters get damaged untill you come ( even with use of gravitons if you travel ).
I have completed whole game ( BPs , missions , areas , locations ) to this point. I understand your words - not watch and you will lure outside units. But you get my point above as spots are small and you can't just stand right at shelters of Safe spots to survive because Mission units land on it.
You can't catch all 3 Safest spots at once to get easiest outcome in Mission. They are "Far". Ancients will already make heavy damage even to safe spots before you come , since Outside units will make your movement slower if you have to run to not get podded.
If there is anything you need to ask , you have all answers above. If i didn't know to drive my ship or survive in planet , i would not say a thing. I can pass whole planet with obsidian which has only repair droid while planet is good crowded. Tau Ceti Part 1 was lots easier on terms of missions which allow us to at least not use gravitons in missions where we need Outside Cover. Most of Tau Ceti part 1 ( Venar ) missions didn't need even outside cover and i did them with 2 people in squad , or even solo.
Note: This mission "Worse to come" has 4 units that land on shelters if squadron membes are 3.
If Squadron members are 4. You need to deal solo with 6-7 units. 5 units in 4-man squad is surely not true. As i saw myself 4 Baiters , 2 horus and 1 shysis in 1 wave. Tell me that any ship can manage to do 6-7 units solo without "Gravitons" ? I know SM diffuclty well , so if you have any questions , read all above metnioned in this post. Many players were nerving how this mission is put. They can get on spots safely , but without Gravitons surely it can't be complete. Please come to Askone and ask players , or any other server if they can complete mission without gravitons. Also , sorry if i seemed not nice in any part of this Post , i just really express whole difficulty of this mission along with other players who can't complete. In 11 fails i had in this mission , i had to use many gravitons if outside units just come there and attack , or if many baiters in mission land. I kept all 4 shelters above 60-70 HP all 11 times with use of Gravitons. If i was cloned into 3 people , it'd be complete , but others who don't have Obsidian couldn't save their shelters like me. Thanks for reading , this is feedback from me and opinions of many Askone tc2 players. 
. Positioning is a really important point here, as you say, if you are killing in a 1 side of spot near defenders, they will probably see you, to avoid that, you have to go to the other side 
