When the Pi debuted, the prospect of a tiny, low-power one-piece 
server was absolutely tantalizing. The facts are impressive but
don't quite live up to early hopes. 

Experience so far with flash devices has been somewhat mixed. On my
"production" machines, which are mostly idle, flash has been simple,
cheap and so far (over four years) fairly reliable. If treated as a
"write once" medium I think it's prudent to use. Buy a bigger flash
device than you think you'll need, let the filesystem grow without
any drastic cleanups and then retire the device when it reaches maybe
two-thirds or three-quarters full. Or, if it becomes noticeably slow,
during writes, implying garbage collection has started.

However, if usage is at all active, such as a test or development
system, the story is different. Three devices have become effectively
unusable on my test machines. One failed read-only, one slowed to the
point of being unusable and another failed utterly, one might say "trash
only", without visible warning. The read-only failure could be have been 
recovered, the slowdown was still usable, fortunately the "trash-only" 
case was expendable but the most disturbing.  

The greatest impediment to USB boot devices is adapter compatibility.
Not all work easily (one of mine won't tolerate other USB devices 
during boot) and there's no definitive guide. Another issue is UASP
support. FreeBSD doesn't have it yet, but the Pi4 does and it makes
a big improvement over bulk-only. No harm in bulk-only, but I don't
want to buy second-rate hardware if I can help it, and it's not easy
to avoid at this point.