I'm not sure Go is saner just because len is an int. Well, maybe, depending on how you look at it. Defining len to be signed int, means the largest valid len is half your address space, which also means half of all possible indexes are always invalid; which makes some things easier.
But it's really that integer arithmetic is not undefined behavior regardless of signedness, that bounds are checked, and that even indexing your slice with an int64 on a 32-bit CPU does the full correct bounds check. In fact, you can use any integer type as an index.
Given all of the above, indexing with a uint or an int is actually indiferent. In that case, the bound check is a single unsigned <len compare (despite the fact that len is signed).
What's really painful, is trying to handle a full 32-bit address space with 32-bit addresses and sizes, like in Wasm; you need 33-bit math. So in a sense, limiting sizes to 31-bit (signed) does help. But at the language level, IMO, the rest matters more.