Nah, even Thick Skin is pretty balanced all things considered. Not only Does Tahm have no real movement speed outside of trying to Initiate (Which, realistically, no on is ever going to do when the same skill can be used for damage if playing top tank or protection if playing support), but activating his Thick skin is actually a GOOD thing for you. Not only does it put it on a cooldown that means either Tahm has to back to base to heal up or risk being killed because his health is so low now from your fight. Either way you win there.
If there's anything on the River King that is a little imbalanced, it's his mana costs. Near all other champions that can do the same jobs he can run out of mana fairly quickly and either have to build at least something to offset that or be very choosy about what kind of spells they're wanting to use and when. Tahm on the other hand kinda doesn't need to worry about Mana at all. He's no where near as bad as, say, Mordekaiser or the Energy champions in terms of just being unable to deal with how often he can use his spells, but he could easily build nothing towards mana and still be a terror to deal with.
