Yeah, pretty underwhelming bonus. 1% HP / 1% damage would be better, or standard skill cooldown decrease... Still, it is royal pain to level up 4*-5*, so those may be worth feeding themselves.

~Fixed bug where Downfall ability stuns enemy units (silly KV-2)
Wait, downfal did NOT suppose to AoE stun? But, it was right there in description....
Or was it self-stun from enemies Downfall?