Trying to compile Super Mario Bros using the SMBDIS assembly and ASM6 results in an error stating that the label "MushLExit" is undefined (it actually just gives a line number, but that seems to be the problem).
Second, after adding the label where it should be (in front of the RTS instruction in PullyRopeObject, from what I can gather), the resulting PRG ROM is 2 bytes too short.
Third, after adding a .org $FFFA statement before the interrupt vectors, the game works (at least the first two levels which I played), but is not the same as the original game. HxD reveals that there are thousands of differences.
These issues should probably be fixed.