Not in my experience. I've got several bikes with external bottom brackets and have had zero problems with them. My commuter bike has 16,000 miles on the current external bottom bracket in all kinds of weather and I don't run fenders all year long (mostly from November to April) so they had a lot of stuff thrown at them. I don't know what other people are doing to their bottom brackets but sealed units of any flavor...square, ISIS, Octalink, and external...have all had long service duty cycles for me. I've never had one fail. I've replaced lots of them but that's during upgrades, never for failure.