It is allowed by the software, but not all possible hardware will work well, and all possible hardware in the case of USB external drives is very much a moving target, unlike standard SATA and SAS connections.
Many hardware implementations of USB external drives have exhibited one or more of these problems:
Non-unique identifiers, so Unraid can't determine which disk is supposed to be assigned to which slot in the array.
Presenting drives with a non-standard size. This won't matter if not using parity, but parity can't be smaller than any data disk, not even by one byte.
Not passing SMART information, so drive health can't be monitored by Unraid.
Disconnects which causes a disk to be out-of-sync with parity. When this happens the disk is "kicked out" and has to be rebuilt. This will take several hours, depending on size. And it may happen frequently. This is why I said it might be OK without parity. But of course, without parity, you can't recover a failed disk.
Probably others I am not recalling or not aware of.
Another thing that is asked about is trying to use an external enclosure with a single USB connection for multiple disks. Parity operations are intended to work in parallel and if they can't performance is significantly affected.
Much of this is unimportant for a single external drive that often isn't even connected all the time, which is the intended market for these designs.