I think the early Shimano external BBs (LX580, XT760 etc.) didn't last very long but the newer ones despite looking exactly the same last much better. My modern Tange square taper only lasted 2 years before developing a bit of play. My UN54 has done over 10,000 road commuting miles though and is still perfect.
If you think external bottom brackets are bad the new generation of bb-30 etc push in brackets are utter plop
Some people are only getting a few rides out of these.....Another bike industy bloomer!
I thought the whole point of the BB30 format was to increase stifness beyond the 24mm HTIIs and alike but also allow large internal bearings like square taper so also improving reliability.
I wonder how many times can you press those bearings in and out before things get a little loose. I wouldnt want to press my headset cups in and out every few rides especially with an ally frame.