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A big problem is that C doesn't make you stick to the representation.

If you accidentally subtract and go into 'negative' numbers, using an unsigned in C will just happily give you the wrong answer.

Now in something like Haskell, it's much more robust advice to make invalid states unrepresentable. But C's compiler and runtime don't typically help you, when you are trying to break your representation.

A couple of things I can think of off the top of my head: Since the compiler knows that certain things are UB using signed offers better (often significantly better) optimization opportunities then using unsigned. Plus, you get to use UBSan to detect bad code.

Here is an example: https://wesmckinney.com/blog/avoid-unsigned-integers/