The enclosed fixtures thing was an issue the CFLs as well, which is where I became aware of it. It’s a similar, but different problem because the ballast overheats. I had several bulbs start flickering on me rather quickly in enclosures, but work serviceably for a few years after I moved them to a cooler socket. I haven’t had the same luck with LEDs, those tend to fail completely if they get too hot and stay dead.
I remember hearing talk of LED compatible dimmers, I don’t remember what the difference was, but I assume when you’re testing dimming, you testing with a dimmer that’s designed to work with LEDs or they’re never going to work well.