I say this as someone who's survived every jungle meta. The tanky-CC-king dominance of S2. The "Invade and delete enemy jungler level 2" meta in S3. The continued "Invade and delete enemy jungler level 2, but with 50% more Zed and Renekton!" meta of s4. It goes on and on.
As a jungle main since S2,
is fine.
His insane late game is off-set by him basically not existing before his transformation. He loses duels to.. just about anyone.. until he's picked one form or the other.
His bases are also generally meh-at-best. He doesn't do damage without building it, which means that he's super dangerous if he gets fed (which is fine), but can be shut down and easily controlled.
A champion being powerful, or killing you, does not make them cancer. Kayn can become a fucking monster, and he's absolutely a threat to entire teams.. But he also has plenty of counterplay (don't stand next to walls; he loses almost all engage potential), and strong early game junglers can easily make his life a living hell.