I've shot various HK rifles and pistol caliber carbines and like them well enough. Now I'm considering buying a rifle, probably A JLD PTR 91 and I'm wandering if there are any durability issues I should know about first.

Durability is gonna cause you problems with these rifles.
(You're gonna spend a fortune on ammo trying to wear one out!)

agreed! sturdy is an understatement.

They are built like a tank.

I have had a HK91 for ten years and my Dad has had one for 24 years and both have never a jam.

Had one for about 20+ years and probably at least 5k down the tube.  Always worked.

The weakest part of the HK9x series rifle is the roller locking plate.  They fail occasionally.  Replacement ones are cheap (less than $20), and the gun will function just fine without one.   The only reason they are there to begin with, is to keep the rollers from falling out of the bolt when you remove the bolt/carrier from the rifle when you field strip it.

The only other draw back is cleaning...  They get dirty, and the breech area is hard to get at.   On the flip side, they will run for thousands of rounds between cleanings and still function 100%.

edited to add:  you will also need to replace extractor springs occasionally, but they are easy to replace, and only cost about $10 each.