I soaked a painted fork before. It didn't mess with the paint at all but it didn't do a great job cleaning up the rust, just kinda changed the color (lighter).

I've found that it works best on chrome, turns chrome new

Tried a rusty chain, it reacted poorly and turned chalky white
I've done aluminum before just because a part had both steel and alu on it. The acid definitely dulls the aluminum but if you don't leave it in too long it won't pit. After the bath just take out your alu polish and shine it back up
Galvanzied steel will definitely react, don't even try it. For me it turns chalky and yellow