I'm not sure what year things got better but I believe they did make changes. We really don't hear much anymore regarding the subject - at least with newer trailers.
I would not hesitate to buy one that's 3~4 years old.
BTW, I've read so many poster claim it's the manufacturers fault Lippert provides defective frame because they demand light and cheap.
That couldn't be farther from the truth. Lippert frames don't fail because they're light, they fail because the workmanship on the welds is horrendous. Manufacturers don't spec welding techniques, that's something that's caused by poor training and lack of QC.