As
@Ridixo notes above drained require targets and significant investment (and, in MP, the ability to actually fight manual battles, which is limited by the battlecard system). The energy used on the tactical op, the chance of a drained being spawned succesfully all represent lost opportunity costs. Where you might chose to spend your energy on generating drained (or rather the chance to generate a drained), I'd use it to rush out another tier1 or tier2 unit, or weaken the drained stack with a ' nuke' op.
Further drained are fragile and cannot mind control until the max rank. In my view they, even massed, would fare poorly against lvl 3 defences+turrets on most races.
I do agree the drained are powerfull, even before the tech tree buff mod or divine legion strat op. But those, again require further investment.
To counter mass drained, wouldn't mass mindless units, long range or high damage, single shot units (trenchers, snipers, Forgotten Golems) work? Alphastrike a single drained, kite, strike again, repeat?
The free mods are a more siginficant issue. I agree that this provides a (perhaps too) powefull reward for converting drained. Especially because you may then invest in upgrading the drained to the most optimal form, which may include mods you normally dont have access to.
It is s a lot of fun to acquire cool drained, but I can understand why it would be considered op.