A few years ago I needed to get new prescription lenses into my glasses. As this usually cost around 200 EUR and I did not want to spend much more money on separate sunglasses, I have tried out photochromic lenses. These have a chemical component which darkens under UV light such that it automatically block out excess sunlight.
At least that is the theory. The problem is that they get triggerd by UV light, not brightness. So when there is shade on the glasses, they won't darken. Take the following example, where I was on a walk and had sun from the right side. You can see how the shadow of the frame has left a brighter line diagonally through the glasses. Also the left part of the lens is not as dark because it was under my hat.
When you turn around, you have partial sunglasses, which suck. Then car windows block the UV light but not the bright visible light such that the chemical compound does not trigger and one is left with regular classes. That also sucks.
So although they might seem appealing, they just totally suck.