Oddly enough, I've only seen Smart Resume working on some of the shows on the HGTV network. I recently recorded a show from there on my DVR, and when I played it back Smart Resume worked OK.
I haven't noticed any problems. Not all networks or shows will support it. Plus, it takes 30-60 minutes after the show to be updated. I generally see it on most major networks for prime-time shows, and often into late night shows (Fallon, Colbert, etc...) Even shows like "Josh Gates Live" on Discovery have it.