The Prizm Trail lens is great for mountain biking and also gravel riding. However if there is heavy tree cover and the light is becoming more limited, then they can be a little too dark. I wore them for Leadville last year and they were absolutely perfect for those conditions. I also really like them on days when riding gravel in sunny conditions.
For heavy tree covered trails, or later in the day rides as shadows grow longer, my new favorite is the Prizm Low Light (Oakley PRIZM™ Lens Technology | Oakley Official US)) I noticed during some of the XC World Cup races last season some of the pro’s, like Kate Courtney, had them on. They haven’t gotten much attention with the Prizm Roads and Trails getting most of the hype. I picked up a set and have been using them a lot this year for XC riding but also for cloudy days on the gravel bike.
Oakley’s photochromic lenses are also solid for mountain biking in variable light conditions. They adjust pretty quickly but don’t have the “Prizm” technology.