There are oddity fixtures out there that have fixed addresses. They are few and far between, but they are out there.
Again, its not a MyDMX or any other controller's issue so much as a fixture/designer issue. If you design a show that has those type of fixtures, you must be ready for those types of issues. If you don't want to deal with those types of issues, then design the show with different fixtures.
Some days I design a show with all Martin fixtures, others all Vari-Lite fixtures, and others still with all Elation fixtures. It is ultimately a preference, cost, and what I need out of the fixtures that determine what I use. Mostly preference, but I can fit custom gobos from a Mac 2k into a VL3500 or visa versa, which is a what I need instance. The cost instance is bang for the buck, which Elation absolutely rules right now.