Structs or unions cannot be part of arrays. Local variables are local in scope, but are stored in the global memory space, and the memory is not reused by other functions. For this reason, I used a few global variables for temporary data, loop counters, and the like which are usually implemented as local variables. Bitfields are not supported. The compiler uses about 22 bytes of RAM for working registers.
Macro syntax is different.
Binary contants are specified with 00000000b rather than B'00000000'. MPC already pre-defines all the internal registers, so there were quite a few conflicts. Support has been fair. In particular, it adds a linker and support for separately-compiled source modules. They are obviously very similar, but will likely each diverge in their own direction in the coming years.