They are - probably more proficient than with some high-level languages. I've used it for embedded stuff, including TI sitara PRU assembly, with great results.
Frontier models can also easily "learn" directly from the manuals; asm is quite easy for them to pick up due to its "flat" (non-structured) nature.
They are proficient at IBM 370 ASM, but need a lot of help, and a professional level disassembler for x64/x86, and in the converse, if you watch HackerNews, AI is poor/expensive for creating compilers, but it has extensive reference materials for MC68K, Itanium, SPARC, and ARM. This is where Gemini and Co-pilot become good co-pilots.