There are a few classes of issues with disassembly tests. As the assembly tests are mainly a sanity check to ensure that the bytecode format is well properly understood, these issues are low priority.
Formatting issues that do not effect semantics (differences in white spacing)
Sometimes more operands are parsed then expected (cause unknown, perhaps padding interpreted as data).
Printing literals with the wrong formatting
Minimum precision annotations
Float printing, FXC uses the standard printf float formatting that is difficult to replicate in C#. Assembly is normalized instead.
There are a few classes of issues with disassembly tests. As the assembly tests are mainly a sanity check to ensure that the bytecode format is well properly understood, these issues are low priority.