While I'm really interested in this for making languages available everywhere (tm), basic things like signed integer overflow being undefined behavior make me question is C is fit for purpose even here.

Since you are generating the C code, you can generate the appropriate checks where needed to avoid such undefined behavior too.

Which is the one thing keeping it in the running.

The problem is this line of reasoning doesn't generally hold up when applied to C codebases (because in practice statistics catches up with you eventually) and C code generators, in effect, happen to contain an admittedly small but rather obfuscated C codebase.

Moreover, you must always include such checks - manually crashing on e.g. null pointers even if you'd be entirely satisfied with a segfault - just to avoid the optimizer potentially going apeshit on your code. Which can have... poor effects on program performance.

If you are writing a code generator, it all depends on the source language. If, for example, your source language doesn't have NULL pointers, then you don't have to waste cycles checking them in C.

Indeed. That's what Nim and Rust (for instance) can do to ease porting to other platforms. Even safety critical applications can be written in C by using tools like FramaC.

https://frama-c.com