I've recently taken to refreshing myself on 6502 assembly in order to write software for the C64. Initially I was torn between choosing learning assembly for the Gameboy and the C64, but I figured development would be more satisfying on the C64 since I would be able to see results on the physical hardware without much hassle. There are also some features of the 6502's instruction set that I like over the Gameboy's CPU such as the index registers being very helpful with working on larger amounts of data and not being restricted to perform comparisons exclusively on the accumulator. I believe the zero page is larger on the C64 as well as not being forced to transfer and execute a subroutine in the zero page just in order to update sprite RAM.
Anyways, if there are any of you that have experience with writing assembly for 8-bit platforms I would love to hear about your experiences with it.
For those who would be interested in learning assembly for an 8-bit platform here are some resources for learning how to do so:
6502 assembly crash course - https://skilldrick.github.io/easy6502/
C64 specific tutorial - https://github.com/petriw/Commodore64Programming
Links to various GB assembly tutorials - http://www.chrisantonellis.com/gameboy/
Platform agnostic tutorial for Z80 assembly - http://www.chibiakumas.com/z80/multiplatform.php
If you want to learn how to program for one of these platforms but you think you can't for whatever reason, it really isn't much harder than writing C. It's probably more tedious than anything, but certainly not difficult to wrap your head around.