"Solar-powered" is a massive overstatement. It would be more accurate to describe them as solar-tweaked.
Reviews report that the Fenix 6X gets about 1% charge in 4 hours if left in full direct angle sun - much less if worn on the wrist at sub-optimal angles in varied lighting conditions such as outdoor areas with trees or if the user is wearing winter clothing with sleeves. In reality, a Fenix 6X user may only be getting 10% of the full charging rate, and a 0.5% total charge/day wouldn't be a surprising real world result.
It's a cute feature and perhaps fun for posers to brag about, but if you are serious about backcountry use, you'd be far wiser to get an Apple Watch
portable power bank which gives you up to 10 full 100% charges.