People keep saying not to use this, and I think that's pretty wrong-headed.

Sure, if you're going to do greenfield software development, and you don't absolutely need to kind of closeness to the metal (not just performance, but direct access) that C offers, then use something else (Rust, Zig, Go ... if you need to compile to native binaries, Java, C# & their myriad of variants if you don't).

But if you have to use C, to extend an existing project, use a specific library, or hook some other system... why not use Cello?

For sure, this is not a tight black-box abstraction, so you should strive to understand the semantics of how it works. Once you do though, it'll make your life a whole lot simpler.