Rule of thumb for doing a programming language: Write the debugger first. Or at least, very, very early. This will inform you of poor choices you've made, and you're going to need a debugger anyway.
At this point, he compiles Jai to C, and the compiler isn't too gigantic yet, so there's not a huge surface area to look for compiler bugs.
Well, for one, you can't just transpile to ANSI C, compile, and end up with sensible debug symbols in your binary. You'll end up with debug symbols for the C code, not the original language. So you end up writing in one language and debugging in another. This is one reason most new languages target LLVM.
Backtraces will sort of work (although the name mangling will leak through), but little else will. You really need to generate DWARF directly in your frontend in order to have an acceptable debugging experience.
Doesn't that depend entirely on how similar your language is to C? ;)