> Are there any big downsides to compiling to C instead of LLVM?
Well, for starters, you have to import all of C's undefined behavior, so it would be basically impossible to implement, e.g., 2's complement signed integer arithmetic.
Strictly speaking it isn't impossible, just much slower since you'd have to implement on software what the hardware does.
In practice though pretty much every compiler that uses that as an optimization has a flag to disable it and chances are you'll be needing to use other system and/or compiler specific stuff anyway.
Well, for starters, you have to import all of C's undefined behavior, so it would be basically impossible to implement, e.g., 2's complement signed integer arithmetic.