I haven't used VMCs at all with OPL yet, but so far real memcards have worked with pretty much everything I've tried. I'm pretty new to this, so I admit I've only tried a few games, but so far they're okay, including a couple that apparently aren't compatible with VMCs.
The one issue I've run into so far is that if you're running FMCB (or maybe just OPL?) off of a memcard, games don't seem to be able to see that memcard slot, even if you swap the FMCB card out for another one. That's a problem if you boot from Port 1 and your game only sees Port 1: it doesn't hurt the game per se, but you won't be able to save or load. It's better to boot from Port 2, so that Port 1 can be used for saving.