A while ago I dabbled a bit in GBA homebrew development. I have to say that many of the modern open source handhelds aren't all that interesting from a homebrew development point of view either.

Devices like the GBA, the SNES etc. have specialized sprite and tile map co-processors that are an interesting challenge to use, especially due to the very limited CPU and memory. Effects like Mode 7 perspective requires you to catch a interrupt to change the affine transform of the tile map at every scanline. That's fun!

Many of the modern devices are just a fast CPU and an RGB framebuffer.