You'd think this is the case but in practice it's actually pretty solid. Right now I've got a nontrivial(opengl + vm language + ssl + http) project with a mix of C/Rust I'm bringing up that runs on:

  x86_64-mscv
  x86_64-osx
  x86_64-linux
  armv7-android
  i686-android
  arm-linux-hf
  armv7-linux-hf

Aside from needing a linker per-platform rustup takes care of all the LLVM stuff, it's really much simpler that I thought it would be.