Flatliners is on Netflix Instant view FYI. Although I've heard those don't always sync up to Rifftrax due to some frame rate or something. And of course if your internet connection causes even 1 second of buffering, you're screwed.
I have only tried it with a few iRiffs (Short Circuit, Frogs, , March of the Penguins, Friday the 13th sequels when they were on Instant, etc.), but I find it works fine. You are right about the buffering, though.