Bicycle Mechanics - Bottom Bracket -Plastic?

I have just bought a UN73-Shimano BB and the left side sleeve is plastic, the LBS says it is very strong. How well will this plastic part stand up in the long run? Apparently all the new BB UN 53, UN73 has plastic sleeve on the LHS.

My instinct tells me this isnt any good, but when I think about it my brain tells me it is OK. It has to transmit a compressive pedaling load from the bearing to the BB shell - I believe the plastic is perfectly adequate for this. I has a very small axial load to stop the BB floating from left to right - if it were any bigger it would make the bearings bind.

The BB on my MTB, unknown shimano P/N, has a plastic left side retainer. No problems with it after 1500 miles. Don't worry.

Most of the Shimano cartridge BB's I've seen have plastic sleeves. I just figured it's the threads on the drive side, on the cartridge itself, that are taking most of the load. That may be an oversimplification, but in any event, I've got probably 4,500 combined miles on three such BB's (one on the pilot end of a tandem) with nary a problem.

The only plastic sleeves I have seen break have been from the tool getting cocked in the sleeve on installation or removal. Make sure it goes in smooth, it is pretty easy to cross thread the thing when you stick it in there. Also disregard the instruction about fully inserting the the right side first. Get the right side started, then start the left side. Then go back and tighten the right side up. Otherwise you may have a hard time getting the left side started.

the one time i dealt with that, i was tourqing it down, and the F***ing thing shattered. Piece of crap.