Here's a
Compatibility List I think the key to the NES Powerpak is that you have to install loopy's mappers over the latest beta mappers (both found at the link above) and that helps out.
Speaking of support I noticed over a year ago when I got my SNES Powerpak that game genie codes didn't work on HiROM games and emailed support and I never heard back. I started a thread on NintendoAge and was told that they got my email and were going to look at it, so yes if you find a bug good luck on getting it fixed as I don't believe either Powerpak have been updated since I got them .
Krikzz mentioned working with mappers on twitter a week or two ago and then he did answer someone that the mappers he was referring to was working with 8-bit mappers so maybe he means NES? It's the only other 8-bit system I can think of that he doesn't support and uses mappers but I could be wrong!