I’m not a heavy Twitch user so maybe I’m wrong but I’m struggling to think of a killer feature of Twitch or an aspect of the overall experience that’s better than YouTube Live videos.
At least with YouTube I can watch ad-free with a Premium plan.
First of all, an Amazon Prime subscription soon won't even remove ads from Amazon Video, so why would it be any different for Twitch?
Second of all, a channel subscription (through Amazon Prime or otherwise) removes ads from that channel. If you want an ad free experience across the whole site, Twitch Turbo is $11.99 USD.
I don't subscribe to any channels because Turbo exists; they probably don't like that.
Discoverability, content creators and community.
Live streaming is a second class citizen on youtube, and as such it is hidden under dropdowns, the live streams aren't always on the top of your followers, live recommendations are worse than useless and the big creators that move to youtube do like Ninja with Mixer, with a big fat paycheck to justify the loss of viewership and community.