The spec is very large, not particularly well written, and is not “total” (in the sense that AMD64 and IA32e and other x86-64 flavors are all subtly different). There are a lot of ways to get it wrong; even XED (the reference decoder from Intel) has bugs.

If I remember correct, the Intel SDM alone is over 3000 pages long.

lol, no. For one Capstone has a lot of bugs (it uses some old version of LLVM as its base) but the whole question of how to decode things is complicated because there are a lot of pitfalls and inconsistencies that different disassemblers handle differently. And what the hardware does is a different question entirely: it may not match the spec, or even other processors with the same ISA.