So I can dash/flash out of Alistar headbutt but not Jayce hammers

Alisters Q doesnt need a target before it activates, press Q and it goes off, so during that combo its possible to evade. As where Jayce in hammer stance, his Q, requires a target to click on first, though i don't know if it's still required for quick cast. Just realize the skills in league interact as if quick cast were off, so im assuming with it on, it wouldn't change the outcome.

Elise Spider Form Q. The tilt.

I don't know if it was somehow changed since the latest patch, but you could always blink/dash out of Jayce hammer Q, or even run out of it if you have enough movespeed. It has always been done by getting away from Jayce's AoE radius before he lands, which cost a lot of kills for me whenever I've tried chasing someone down with hammer Q.