What's all this with USB?
There was a thread on the raid mailing list in January 2018 where someone asked why the wiki said "Do not use USB". The general consensus is that "it is a very bad idea", but nobody could really put their finger on why. Just that bad things seem to happen if you do.
However, it seems likely that the cause is two-fold.
Firstly, the cables are prone to being knocked and jiggling. This can easily trigger a disconnect/reset sequence which, if it happens to occur in the middle of a write can be extremely disruptive.
Secondly, USB is a star topology which can hang up to 128 devices off a single port. Hanging multiple drives off one port is not a good idea, but all too often it is not obvious. Even when drives are plugged directly into the motherboard, they may be connected via an internal hub to a single port. Back in the day your editor built a system for copying CDs. Originally, it had three CDRWs and one hard drive plugged into the two onboard PATA connectors. Performance was abysmal. Then some add-in PATA boards were purchased, and the system reconfigured with only one CDRW per cable. All of a sudden the CDRWs ran at their rated 48x speed or whatever it was, rather than running at less than 1x.
And lastly, many external enclosures are poorly ventilated. Using a drive heavily is likely to overheat and kill it.
So the general conclusion - although there are no hard and fast reasons - is that USB for raid is a bad idea. Don't do it!