Yes but it costs way less compared to others to kill it. And the huge bubbleshield you mentioned is also the problem I was bothered with, but dunno if it changes that much since the fatty itself is enarmous anyway.
GC:
3,63 hp/mass
Chicken:
2,53 hp/mass
Monkey:
2,25 hp/mass
Mega:
2,93 hp/mass
Fatty:
0,45 hp/mass and
0,71 'shieldpoint'/mass
Yes, shield recharges and shit but in practice the Fatty doesn't die over time but in one sudden event, e.g. landpush or snipe.
I had bois also in mind, but that's additional, very expensive, cost. UEF bois however are afaIk the strongest combat bois (If not, feel free to correct me. Not sure there since Cybran-emp and Sera-OC are juicy as well) but I don't think they're a good alternative against other exps if you compare the cost etc.
With bois you just make the cost of defending a fatty basically the same as building two GCs.
Speaking about ingame, I've never seen a fatboy getting countered by land units (except mismicro), but that's exactly the thing. Because it is so easy / cheap to counter with air, there is no need to counter it with land forces. If we both invest the same, you into the fatty + support, I invest into air-shit and a couple of arties. I'd say that 98% of the cases (at least my experience IF the airplayers are equal) the fattie dies with an equal trade (assuming I wasn't stupid and sent 100k mass into your base where your fatty was)