Sort of supports Mac... No one on the current team has any Mac hardware. Myself and a couple others occasionally make unofficial builds but that's been about it for the last year or two.
As far as iOS, as with many open source projects, a major issue is the licensing probably can't pass the App Store rules.
The GPL in particular is fundamentally not compatible with the App Store re distribution clauses.
VLC for iOS being the precedent setting example from a few years back.
To answer your question directly, I'm not sure of cases where Apple will outright reject the initial submission of some OSS app - especially if you aren't totally up front about it. But clearly re VLC they will pull it once it is brought up (also leaving to distributing developer open to liability depending on how unhappy the contributors are)
As far as iOS, as with many open source projects, a major issue is the licensing probably can't pass the App Store rules.