Well the Stuart abilities cost muni and to do the engine shot you have to go pointblank range, which is not possible most of the times. Also it pretty much sucks as AI vehicle compared to luchs/t70, and another thing is that it is slower than those two. People now are forced to go for Stuart, because they need something to hold off all the LVs coming at you.
You mention Puma, which if played correctly can kill Sherman or T34/76. So you don't need to tech up and just wait for your Command Panther lategame.
it would op if it wouldne cost anything...this isnt an arguement.
why it should possible to dmg engine with stuart? it is really fast...and can flank easily..it is really usefull in midgame.
USF dont really have problem with LV....because of the cheapest AT unit in game: double zook pios.
puma is a victime for well handeled stuart...because stuart win with the stun abilty.