Variable names likely had to be uppercase, and there likely was a limit on variable name length (either hard or of the “you can use lowercase/more than eight, but we will ignore case/all but the first eight” variety).
I wrote a bunch of assemblers, each one better than the last, ending in the one that Atari used for Atari ST, 7800 and Jaguar development. It's the assembler included in the 7800 tools archive. Someone cleaned it up a bunch and put it on github a few years ago.