If a XM800 farts at you in a MBT, your barrel is gone (unless you are in a Russian tank, for some reason Russian cannon barrels and tracks are much harder to kill…). Then it just swoops in the side and kills you. They are also completely silent, Fox is probably the loudest vehicle in the game.
Fox and ZA35 are highly skill dependent vehicles where a good player will do extremely well in them, but an average player will do poorly in them. Something like the Turm III is going to be extremely good in a good players hands, and still very good in an average players hands.