If you lock 1 Comet/Panther for player, so it will be difficult to fight for example against IS2.
And with only one Panther you cannot effectively flank.
IS2 is really bad, you don´t need more than one Panther to fight it off really..
As for the comet, this tank is the best non doctrinal tank in the game and can fight infantry and armored vehicles effectively. It can easily snipe its weapon team counters with WP that has insane range and can actually kill lol. There is no drawback to getting this unit, it can do it all.
The problem is, most things UKF have are ranging in the region of pretty decent to pretty oke, it is the rest that is just overperforming hard that makes this faction utter BS. Nerfing must be done carefully. If nerfing is not done carefully, we might put UKF in the position USF was in once before the Terminator Riflemen Buffs. USF was hopeless back then and I fear UKF will be the same if the nerf hammer hits them good.