My pals seem to have had nothing but trouble with external BBs on road bikes. I shudder to imagine the lifetime on MTBs with all that mud.
Cartridge BBs are OK and I have a couple of UN72 which have lasted over 20,000 miles. However, to get it all to fit in a cartridge the bearings are smaller than a cup and cone, which has 11x 1/4" balls. That's fine on a solo bike, but tandems have much higher bearing loadings and cartridge BBs only last a couple of thousand, whereas the cup and cone ones last longer in terms of bearing wear.
That's why they turned to external again
Moving to ISIS/Hollowtech(Octalink) made the cartridge system strain even further as the shaft got thicker balls shrank.
So they moved them outside to use bigger balls or made them fit in the frame itself (BBxx stuff, way to modern for me to bother to know it's name)
I must say though, I'm disappointed as my original UN90 has started to get some play, still very smooth running though.