Hi Miffy, and welcome to the Thorn Cycling Forum!
I deliberately chose Outboard/external-bearing (I'll use the terms interchangeably) BBs for my original Sherpa and again on my later Nomad. I'm a bit pressed for time, so will answer in bullet points...
• External-bearing BBs use seals (shields, in most cases rather than the more robust seals typical of Conrad-type cartridge bearings). What this means is they really require no maintenance. You use them and use them up, then replace. It is possible to pry off the shields and inject some grease -- you can even knock out the bearings and press in replacements -- but as a rule, they are maintenance-free.
• Many external-bearing BB's use less than full bearing complements, so there are fewer ball bearings to share the load. To compound this, the balls are typically smaller in size. Sometimes, they aren't greased very well. These can all lead to a shorter bearing life, particularly if all are present.
• In my experience, "most" OBBs are "good enough" for general use and good service life provided they are not installed with excessive preload on the bearings(!). When installing the OBB, the left side has a little plastic preload adjuster that can easily be turned tightly, placing a lateral load on the bearings, causing them to wear quickly. The adjusting nut should be installed with only enough tension to take up or prevent any excessive play. A lot of people with failed OBBs cranked this adjuster down and paid for their mistake in a short service life.
• OBBs seem to do fine for the majority of people when installed correctly. They need to have parallel BB faces, so a refacing might be in order, and they need to be installed without excessive preload. Do those, and a reasonable service life can be expected. If they are poorly adjusted at installation, see a lot of high-torque use, dunking, or the bike lays on its side a lot in heavy weather, they tend not to last as long as their more conventional brethren.
• As a general rule, Internal-bearing bottom brackets seem to last longer in severe or touring service because the bearings are generally larger in diameter and/or more numerous, and the seals are a bit better, being shielded by the edge of the BB shell in part.
That said, after two Shimanos that failed for me, I bought a Phil Wood OBB and have been very happy. It has so far proven to be very durable (as have their IBB cartridges on my bikes) and I expect a long service life from it. It is well sealed, filled with premium grease, the bearings are supported by robust stainless cups, and the bearings themselves are objectively better.
Hope this helps.
Best,
Dan.
