The frame is in reality pretty strong, partially due to the 1/10" thick aluminum baffle. I've owned countless Lambdas and aside from a few broken plastic cross sections there has never been any damage to the frames and I have the habit of buying badly treated sets to give them a new lease of life.
Compared to the faultless Stax QC. There have been some design issues on some models but a Lambda Pro made in 1982 sounds exactly the same as a unit made in 1990.