I mean, it’s not hard as such, the encodings of some instruction sets are just ass, with 32- and 64-bit x86 as the foremost example and Thumb-2 not far behind it. Also, if you’re dynamically patching existing code, you’ll have to contend with both modern OSes (especially “hardening” patches thereto) making your life harder in bespoke incompatible ways (see: most of libffi) and modern CPUs being very buggy around self-modifying code. Other than that, it just takes quite a bit of tedious but straightforward work to get anywhere.