We do need to solve the lack of hashing libraries in Motoko. I remember @rossberg himself saying that the memory allocation behind the scenes for Nat was highly optimised and the potential savings using bounded Nat types was not signifiant in most cases.
Expect for situations when binary manipulations are necessary I recommend to stick to Nat.
Edit: Found the original post (What is the maximum value of Nat64, Nat32, Nat16, Nat8, Int64, Int32, Int16, and Int8?)