Microsoft MASM is very close to Intel MASM86; in general, Intel-targeted programs should compile with little change. DRI's 8086 assembler, on the other hand, is very different, as are many of the older third-party assemblers.
One thing that I remember from the ISIS-II ASM86 was that it was incredibly slow. ISTR that it started by reading in an ASCII file defining each of the machine instructions. I never used the iRMX version.