I'm in a filtered view doing triage on my iPad. If I make a mistake with a rating the photo disappears from view and it's hard to fix. It looks like rating changes DO impact the sort by modified view (something I've been confused by in the past) so I can probably find and fix it with a few steps.
Honestly without being an jerk, I think this needs to be on a backlog of basic functionality that is high priority. I'm not entirely sure how to define it but if a rating change wasn't undoable on PC we'd be "shook". It should work on iOS also. As well as all of the other missing fundamental stuff.