Long ones may be vulnerable to breaking, when handling the bike folded.
for shipping and being folded frequently..
Dont know about yours, or the generalization:
What's with all the short and undersized rear fenders that come with most of the bikes out there?
Brompton, offering specifics, makes a flap of rubber, molded
so that it sticks out to catch wheel spray, on the back.
and a long flap of LDPE, on the front.
maybe a milk jug can provide some mudflaps added to improve
your particular situation.