I think people who don't like Reaper's MIDI either a) have had it crash on them with a lot of VSTi's or b) are whining cuz they're naked and helpless without the Cubase drum editor
I've always programmed drums using the piano roll, and I have no problem with it personally! However, one other thing that's annoying about MIDI in Reaper is that the regions sometimes don't quite behave as individual regions that start and stop where the edges are trimmed to; most notably, sometimes when I try to drag an 8-bar drum region from one part of a song over to another (but the second is after a tempo change), sliding the region over
near where the tempo change occurs will actually shift the notes within the region, which is fucking maddening.
It's a rare problem though, and doesn't always occur, so I deal!