I reproof my Carradice panniers with Barbour jacket was and a hairdyer. I bought them in 1995 and used them almost every day for 15 years and now pull them out for bigger loads and tours. They do suffer from wear and tear, esp wear along crease lines but are far tougher than nylon. I have patched mine a couple of times.
PU-coated nylon is waterproof for as long as the coating remains intact. This is one of the first things to wear out and once worn, they loose all water proofness.