From my own experience with alari XL/XE I can say that Illegal opcodes ain`t called "illegal" for nothing. Even if they work on 4 machines, on 5th they will crash. In 6502 CPU case it`s mainly because there were many different models produced,so it could be the same with 6507. I used illegals in my game for soft sprite routine, and it worked perfectly on my machine, but then I sent it to competition and the effect was like this: