Clearly YMMV as to whether or not you can support a kmalloc()-style API. If you can, allocators will be great for you, and if not they are worthless.
I didn't mean to imply Rust has one-size-fits-all solutions to these problems. Rust provides a lot of solutions to various problems, and it's up to you as the Rust developer to figure out which ones are applicable to your problem at hand.