Even by wehraboo standards you're quite something. If Guards were op people would be playing only Guard doctrines same way Overwatch was in every game thanks to JLI. Thats not the case.
Guards can be actually OP and dont be seen quite often.
1st logic fallacy there: correlation does not mean causation.
I can name a simple counter example about this. Combat engineers must be OP, since they are present in EVERY SU game, sometimes even two or three squads present at the same time.
The point is, guards can be OPish because they can do multiple jobs quite effectively, more than other factions variants or even than other alternatives in the same faction, because they are quite durable by themselves as a premium generalist inf squad they are.
Early game until almost midgame can be played with guards safely. After that, when multiple mediums, heavy indirect fire and AI specialist squads are present, guards stop shining (for ovbious reasons, otherwishe it will be buttfuck OP). Therefore guards can (or can not) be OP without being spammed in every game. Also commander pick is a important detail here, not mentioned.