[ajenner]

Microcode instruction sets have different engineering trade-offs to the user-visible ISA. In microcode, memory bandwidth isn't such an issue so microcode instructions can be relatively wide (21 bits compared to 8 for the 8086).

The microcode can also be relatively difficult to write. For example, in the 8086 microcode I saw one place where there is a "DEC2 tmpc" microinstruction (subtract 2 from tmpc), then tmpc is loaded after that, after which the correct result is available (this makes sense when you think about how the ALU works on the chip, but in any normal ISA you have to load the values into the operands before you perform operations on them).

There's nothing in the 8086 microcode which creates any temporary undetermined states as far as I can tell but there may be combinations of microinstructions which could create a race condition.