Nope, never seen 'em, but I would still be on the look out for water getting in there, along with break down of the grease in there.
The cap on that end may do well, but the seal on the other end would be my long term concern.
Come to think of it, I don't recall water ever getting into mine thru the cap end...
I posted a while back about when I jacked up my trailer and gave each wheel a spin, and found one to be grinding - no other sign. I broke it down, the grease still looked good, but one of the roller bearings had broken in half, one half was missing, the other half was just tumbling around somewhat in there. No excessive heat, no other warning.
No substitute for occasional inspections - jack it up, spin each way, push and pull on it, keep lubed as needed, and then you can trailer with confidence!
I also know a guy who found a nasty cut on the inside of a trailer tire - don't forget the tires!