The only downside is that fish has no way to allow atuin to override the autosuggestions as you type. We can get that in zsh but fish doesn't expose that.
I've tried it with fish and found that, for me, it didn't add much and wasn't worth the switch, but it is arguably slightly better than fish's built-in history; so I'd say only bother if you particularly love learning new things of this type.
Fish + Atuin (with up arrow disabled) + Starship is what I use on every platform I can