The first thing I noticed when I tried him was: "Aren't these cooldowns awfully high on his skills?"
(Q is 7/6.5/6/5.5/5, W is 13/12/11/10/9, E is 21/19/17/15/13 and R is 120/100/80)
Even late game, a couple of these, despite what it would mean for them to be shorter, are pretty high. Meaning he almost has to build CDR no matter his form. (The cooldowns are balanced around the effect, Q being a slight hop, W being a large AoE slow, E being a heal and wall crawl, R being his ultimate. So I'm not saying they don't make sense for what they are, only that overall he doesn't have his skill overall cooldown going for him in regards to his SA form.)
Then came the mana costs. He wipes his pool quick since he relies heavily on skills.
Then I noticed that he doesn't actually have a passive until he form changes, meaning his early-game "assassinations" aren't going to go off without a hitch, because he doesn't have the dps or the survivability for it innately. The thing he has going for him in SA form is mobility. There should be something to how he plays as a SA that makes him that backline killer other than placement changer via his E. (He can theoretically duck into a wall and just walk around to their backline since walls are not stopping him.)
His build suggestion also is a big hiccup. It suggests Rhaast-durability AD items if you are laning and SA if you are in the jungle. The suggested build changes when you form change too, so new players are going to be scratching their heads trying to get a grasp of what to build.
And back to his Passive as mentioned in other posts... magic damage is really a hiccup for him. It's really not doing him any favors in SA form. The only thing I can think to salvage it's overall effect is to make it true damage, even if that meant lowering the %... but that also shifts players into that tank-killing thing the other form wants to be. It's just difficult to really say what would even be better since he's such a new champ. Late game full build in Practice Tool, I was at 44% conversion. His skills didn't seem to convert the dps typing either or something was up.
Something is hinky about the design that feels like Rhaast was the balance point and SA was an afterthought.
What he does do well on the other hand:
Waveclear. :|
That's all I really got. Maybe make his R let him hop further out of an enemy when you ulti them in SA form. Give him an escape since he probably just all-in'd it and is squish.