I've been slowed more than once by an Irelia, it's just that Irelia players are quite good at keeping theyr health level either the same or just a bit lower than the enemy, which is pretty much the only thing you need to learn to play her. And yes, it is an unfair mechanic since it actually means that when you are wining you automatically lose, but it's still better than having the ability to chain a slow into a suppress while also having 2 health bar and having high base damage with good scaling in both ap and hp.
Irelia might not be the most balanced champion (which is ironic since she's ionian, and Ionia values balance over everything else), but Tahm also has quite a toxic kit since he's immune to poke but can't be beaten in a prolonged fight since he will place 3 stacks on you, stun you and eat you to chuink you for more than twice the damage he's taken, and he can do this while also permanently cc'ing you so you can't get away.
p.s. sorry for the late answer, lately I hardly log to the boards ^^'.