Open Water Goggle Lenses

I think the answer is... it depends, mainly on conditions. I've had zero issues with my Roka googles and if you scroll down you'll see they have examples of the different tints in different conditions compared to what it would look like without the mirror.

https://www.roka.com/collections/goggles/products/r1-goggle

I generally use tinted lenses. There tends to be a lot of glare on the water when I swim (races and a.m. training) so the tinted lenses help.

The main issue with tinted lenses I can think of is that you DON'T want to wear them if light conditions are poor. I'd always have two pairs handy for racing--one clear and one tinted. I've done a few races where I started in an early wave on a cloudy morning and really wouldn't have been able to see adequately with tinted lenses. Also, occasionally it can be a problem if there are different colors of buoys for different distances being done on the same day. Not sure about others but I have trouble distinguishing some colors while wearing tinted goggles.

I agree with Hot Runner about having two sets of goggles. On my last race just a few weeks ago, the used my dark goggles and regretted that decision. It was cloudy and I couldn't see as well as I would like. I ended up following a step that ran along the man-made lake, and then the rope on the way back. I didn't do much sighting in that race. And mirrored goggles may be necessary if you're swimming when the sun is rising and in that general direction.

https://www.youtube.com/watch?v=CViP0yizt8c