I agree that relying on the spring to move a derailleur in high stress situations is less certain that the solid connection a cable provides. I certainly remember waaaaaiiiiiting for a front shift in the middle of a hill because the fd's spring couldn't quite overcome the chain tension and the old-style flat chainrings did nothing to help. Benefits notwithstanding, Sun Tour's reverse action fd never became standard and was never adopted by any other maker.

I have no experience with Rapid Rise rear derailleurs but Sheldon Brown was a strong advocate of them. As he noted, rear shifting isn't done on the high tension part of the chain so it's less sluggish under load. It's easier on the drivetrain since the Rapid Rise allows the shift to occur when the release teeth are positioned properly since the spring won't force the shift the way a hard cable pull will.