The DPCM frequency isn't a problem (it's highest rate is pretty good), but yeah the delta modulation smoothes out all sharp edges. It'd be a good idea to run a low-pass filter on a sample before converting to delta format.

Using DPCM steals CPU cycles, so that's why it's usually not used in cases where CPU timing is really tight. That, and I'd imagine back then not very many people had the equipment to make samples. I'd almost think that's the case for SNES even, it seems like many games all use the same samples.

Rad Racer 2 also does the $4011 drum popping sound (definitely only one or 2 writes per frame, because that game is very timing-dependant).