It depends on their C API. Ruby doesn't really have one, it just has the exposed internals of the vm, and assumptions that can be made about those which are normally true. That includes type assumptions like VALUE being an integer that you can switch on, and _some_ values such as Qfalse and Qnil being constants that can be used in such switch statements, but it also includes things like being able to have a custom GC mark function for a C data structure.
It's very hard to produce another Ruby implementation without breaking some of those assumptions, but we can hopefully get it down to a small set so the changes needed are minimal.
There are other things like throwing Ruby exceptions across C library boundaries which may work in MRI, but whose behaviour may not be intentional and could already have subtle problems, and I don't think we should be aiming for compatibility in those areas at all. Finding and fixing things like that is normally a good thing to do anyway.
