Well technically with UUIDs you could get collision if you're using version 4 (though the chances are tiny, so it's probably not worth checking for collisions). I agree UUIDs are probably a better fit for most problems, rather than prefixing the id. Sometimes it's nice to be able to eyeball a key though and know which system it is for (stripe uses this for dev or live keys for example so they can be easily distinguished), so there are use cases for prefixes.