I have a few with plastic. When I was researching them I believe I read a post from BFG that said they had never had a failure of the plastic adjusters.
We have the acetal (plastic) ones in stock and it's held up great on my personal AR over the last year. Quality piece of kit for sure... Hells Canyon Armory | Slings & Sling Mounts
I have one with plastic adjusters, I just noticed the last time that I cleaned it that one of the adjusters is broke, been meaning to give them a call and see if they will send out another one.