The bigger downside is nuisance tripping because the breaker doesn't like electrical noise emitted by some device or another in your house. I don't have any AFCIs but I've heard it's pretty common.
You'd rather have one circuit go out instead of the whole house, and if all you have is the one AFCI breaker you're going to have a much harder time identifying what device(s) is the culprit.
I've mostly used them on new circuits, but have put them on a few of my older circuits too. For my small sample size, I've yet to have one nuisance trip.
Yeah I've replaced all of the breakers (to living areas) in my 90s home with AFCI ones. Haven't had any nuisance trips. If I had "nuisance trips" I'd be more worried about my wiring than about my breakers...
You'd rather have one circuit go out instead of the whole house, and if all you have is the one AFCI breaker you're going to have a much harder time identifying what device(s) is the culprit.