The x86 front-end is work in progress. We're targeting the 8086 instruction set now and leaving i386 and x86-64 for later. The front-end is currently being maintained by Pekka.
Here are some things that are known to be missing from the x86 decoder (ask Pekka for details):
- SIB byte decoding
- Operand size override prefix decoding
- x87 instruction decoding
- REX prefix decoding