"Break your UX" is a vast exaggeration. I'm not talking about phone number fields, to be clear. I'm talking about like numeric amounts, with maybe a negative or decimal point. It's literally just [0-9\.-] but that still requires JS to limit inputs to that.
It doesn't prevent the bad input, it only validates it on submission. Therefore it's on the user to fix it, and if it's from a zero-width space, it's literally invisible. Non-technical users will have no idea what to look for to fix it (i.e. use your arrow keys cursor to see at which character the arrow key doesn't shift visually, and there's the ZWSP). It happens way too often.
