Don't show an ad when a channel begins to host
Overall, ads on Twitch are reasonable and not too intrusive, but this one quirk makes it more difficult for small streamers to support each other.
If someone ends their stream by hosting mine, I would like to be able to thank them and welcome them and any viewers who came along with them. But at the moment that I receive the host notification, I have to assume that none of them can see me because an ad just started playing for them. And if I host another streamer at the conclusion of my stream, my viewers and I don't get to see their reaction unless without pre-loading their stream, or watching the VOD later, or enabling an adblocker.
Revenue from host-transition ads must be next to nothing, and this does not seem like something that is done to nudge people towards paying for Turbo because the hosted streamer obviously cannot assume that most of the viewers coming over to their channel have Turbo.
No ads ever! Youtube is better.
This is a big one for me. The only thing Twitch really has going for it over YouTube is community. If hosts are functionally useless because the a bunch (not even all, because subs are excluded) of the incoming raid/host gets a pre-roll, then a live response to it is really not functionally possible. You've got to say something to the incoming subs, but be fully prepared to say all of it over again in 30 seconds or whenever the pre-roll ends for the incoming non-subs.