I think most people here would agree that the Fatboy, as it currently is, has some problems compared to the other Experimentals.
Therefore I want to describe what in my opinion the problem with the Fatboy is and where I see a solution. Of course, this is just my point of view and constructive criticism is welcome.
What makes the fatboy special?
Unlike other Exps, the Fatboy is a siege unit. So the fatboy has no immediate impact and needs time to get this impact in a game.
That's why it doesn't make sense to rush a Fatboy because even if you do (and sacrifice your eco to do it) it can't finish the game.
That in and of itself is not a problem. It's just an option that UEF doesn't have to end a game quickly compared to the other factions.
So what's the problem?
What's critical about Fatboy is that it's also easy to counter.
And here we can also find the reason why it is too difficult to balance the Fatboy properly. If there are effective counters, the opponents (if they are not losing the game anyway) have the opportunity to build the counters. And since the Fatboy is a siege unit, you give the enemy all the time to do this. (Arty, TMLs, Air Snipe, etc.). So the Fatboy becomes useless.
On the other hand, if you remove the counter or weaken them, the Fatboy will be overpowered very quickly because, well.... no usefull counter.
So it seems like an impossible balancing act between absolutely OP and absolutely useless.
And what is the solution?
But there is another way to balance units. And this is actually already known and often used in the game. It's time!
Basically, most units (some units are affected more and some less) only have a limited time window in which they can be used meaningfully before they get countered by other units.
The Fatboy can obviously also be countered (there has to be this option!).
But, as one of the more expensive Exps, it can only appear on the battlefield at a time when players have strong Eco and BP to build the counters quite easily and the game is already in the hot final phase.
So if you also add that the Fatboy moves slowly and need some time to get value, the earliest point in time at which you can build the Fatboy typically is already behind the point where it would still have been useful.
You can compare the Fatboy a little with T2 MMLs, which you can only build when Ravanger and Sniper Bots are already on the field.
The solution would therefore be to move the earliest time at which you can build the Fatboy forward and optional move the latest point in time at which it can still be used backwards, so that there is a time window where the fatboy actually is usefull.
And to get the Fatboy earlier, costs obviously need to be reduced significantly. Ideally the mass cost of an ML and maybe slightly below.
This way the Fatboy can be on the field much earlier and would actually have the time it needs to get his value. (And as a bonus, UEF would finally have a reasonable counter to sniper bots.)
Of course, the fatboy should also be nerfed accordingly, by lowering his DPS. However, I wouldn't reduce the HP any further.
But I think the most important factor will be to move the time from which you can have the fatboy significantly forward.
Currently, the Fatboy arrives on the field as one of the later Exps. If you consider the fact that the Fatboy moves slowly and takes a long time to get his value, it becomes obvious why the Fatboy hardly ever has a decisive influence on a game. There is simply no real time window for it because the moment the fatboy could be built it is already obsolete.
So the solution is to move the time from where you realistically can get a fatboy and the point from which it loses its use in such a way that there is an actual time window in which the unit can have a decisive influence on the game.
This means that the costs should be reduced and, if necessary, further options could be consider to extend the usefulness of the fatboy.