Such a functions are exported to a public kernel API sometimes. So any kernel developer could call them. It make unavoidable a mistake of a programmer, when some of those functions was called with an unknown at compile time integer value. It is not a bug as it is, code would compile and work nevertheless, code would be bigger that it could be, maybe slower, that's all. But reviewer should be finding such a misuse of API.
It is just one example, linux kernel uses a lot of tricks and exposes it via APIs, reviewer must be aware of all of them and to check everything by itself.
Compare it with Rust. In rust one can encode a lot more limits on the right use of an API than in C. Encode and enforce. Even when rust sometimes cannot do it, rust have nice macros, instead of C's macro-horror. So a lot of mistakes that could be made with C, could be avoided with Rust. So it would be easy to review code, it would take less time from a reviewer to review code.
Reading other people's code is boring. Linus stated once that he got around that by retyping the code while reviewing, which is a good technique.
One danger is of course the change in culture and politics should Rust actually become prevalent. But there is still BSD.