Originally Posted by
grumpus
I used to use F.A.G. plastic bottom brackets, never had a problem with them. I've also had some Shimano? bearings with a plastic NDS cup that were a bit fragile to work with but functioned just fine. As long as the threads in the frame are good (and you grease them) I wouldn't worry about plastic cups.
You can be fine but plastic is a poor choice and have had to deal with a few cracked cups. We did get everything out in the end but it is not the place for it. I don't think you are really saving a whole lot from aluminum or steel vs. plastic and typically with plastic the quality of everything is compromised but back in the day a UN-55 which lasted a long time isn't super expensive and the UN-300s today are maybe cheaper.