As far as i can tell they have trouble with sustained satisfaction of multiple constraints, and asm has more of that than higher level languages. (An old Boss once said his record for bug density was in asm: he'd written 3 bugs in a single opcode)
I agree with this. Just the need to keep track of stack, flags and ad-hoc register allocations is something I’ve found they really struggle with. I think this may be why it does so much better at porting from one architecture to another - but even then I’ve seen it have problems with e.g. m68k assembly, where the rules for which moves affect flags are different from, say, x86.