By "landmine" I mean situations where it gets very unpleasant to use because the data isn't guaranteed to be close enough to fit into one of those pointers. Even if it's true when the code is first written, minor updates might change that. I don't think something like a borrow checker can do much to help with that.
Let me put it this way: 16 bit indices are already rarely worth it. 16 bit pointers are 10x the trouble for 20% more situational coverage. It's not worth it. They are not similar levels of useful/terrible.
Indices all go in the same allocation, which makes a big difference. You're just putting a max size on your arrays, instead of making your memory allocations a lot more complicated. And if you need bigger, you'll know in a much more straightforward and deterministic way. It also doesn't require the same support from the language, whether that's explicit small pointer support or support for turning integers into pointers.
