Easy, folks. No need to get ugly...
In any case, there may be some odd compatibility issues here and there, but I don't know a card out there that DOESN'T. That being said, I think the primary issues in DHJ stem from the speed of the MicroSD, and not from something the R4 team has or hasn't done. My guess is that both of those games use some special high-speed memory, and instead of loading all assets at once, they are constantly streaming from the card when racers are on-screen. It would explain a lot, as the original card would be able to access fast enough, but the MicroSD would not be able to load all the data in time to keep the speed up.
For the record, I'm using a Patriot 1GB. I felt the need to update that entry on scorpei's wiki, as it too gives slowdowns. That being said, I see no other issues with the games I've tried so far.
Honestly, it works so well, I kind of feel bad about it. I have 15 legit DS games, and I'm sure my DS buying habits are sure to take a hit at this point. I'll probably still buy them, but methinks I'll wait until they're cheaper. In any case, it's been worth it solely to figure out what games I DON'T want. Sonic Rush, Yoshi's Island DS, I'm looking at you...
It would be nice to see some other fixes, and I WAS secretly hoping there'd be more to the update than skinning, but that being said, the card is top-notch as it is. Compatibility is far higher than, say, a PS2 softmod with HDLoader.
Now, if anyone can tell me how to get the nesDS WIP to save on the R4, all will be right with the world.
Sarge out.