That's actually not the issue with SA Kayn at all.
With your wall tricks, it's actually fairly easy to flank and get in from the side or back when a teamfight is engaged. You shouldn't be going straight ahead first.
The problem is it takes a lot of effort to kill an ADC lategame - and if they buy a GA, or have a shielding support, you're basically screwed.
He can actually deal quite a significant amount of AOE damage too, though it's fairly hard to pull that off well.
edit: also btw, RE: his passive, it actually works in a very unintuitive way. There's two bars that fill up individually, and you only see the one that's most full. You transform whenever one of them is full. That's why you always transform into the last enemy - because someone will always be the last one to fill one of the bars, and once you achieve a form, it means you succeed in filling that bar completely first.
Combined with the fact that early game orbs do next to nothing and the amount each orb gives you towards a transformation scales up very fast with game time, it can certainly feel like that, though - one duel with a ranged opponent between 10 to 15 minutes could certainly fill up the blue kayn bar 100%...
edit2: if there's an issue with Darkin Kayn being so much better, imo it's just that Darkin Kayn is a bit too good, in particular he does too much damage to everyone. That should be SA's job, after all.