In practice, one of the difficulties in getting _clang_ to assemble the Linux kernel (as opposed to GNU `as` aka GAS), was having clang implement support for "fragments" in more places.
There were a few cases IIRC around usage of the `.` operator which means something to the effect of "the current point in the program." It can be used in complex expressions, and sometimes resolving those requires multiple passes. So supporting GAS compatible syntax in more than just the basic cases forces the architecture of your assembler to be multi-pass.
https://eli.thegreenplace.net/2013/01/03/assembler-relaxatio...
There were a few cases IIRC around usage of the `.` operator which means something to the effect of "the current point in the program." It can be used in complex expressions, and sometimes resolving those requires multiple passes. So supporting GAS compatible syntax in more than just the basic cases forces the architecture of your assembler to be multi-pass.