Yup, my wife went all out and got one of those UV sanitizer boxes[1] too along with the Under Armour face masks[2]. Their instructions say to wash it but it's convenient to just place in a box. We have also been cleaning other items in there including baby toys.
I can understand how UV light can sterilize hard surfaces like a cell phone, but how does it sterilize multi-layer fabrics where the virus may be shaded from the UV light by the fabric?
And that is called the shadow effect. The UV-C doesn't much make it into the plastic statically-charged inner layer of the mask.
In May I built a 5-ft long 24x24" cross-section sealed plywood box with a small (6x6") MERV13 filtered inlet opening and an auto-detailing fogger filled with 3% hydrogen peroxide at one end. A center chamber with a web of small hooks on a wire frame for the N95 masks, mirror (plexiglass) glued on the walls of the center chamber to bounce the UV-C around and an 18" long Blade UV-C (254nm) lamp that peers in through a slot on the top of the chamber. On the far end I used a ~30cfm duct fan to pull the the hydrogen peroxide fog through the chamber and masks and a flexible duct to take the exhaust outside my garage. I used UV-C dosimeter disks to tune the amount of time needed for proper UV-C exposure (about 5 mins or so). I put the masks in it, turn on the Blade UV-C lamp, turn on the fogger and let it go for about an hour. I then turn off the fogger and let the fan pull fresh air through for another hour or so. It's not pretty but I think it is effective.
Their website states and shows that face masks are items you can sanitize. They also have a testimonial from a health care worker that uses it for masks. Neither go into details but their FAQ page says "UV-C light only sanitizates surfaces it can reach" so that is a fair question you are asking they should address.
I'm still not totally certain how well it really works, but I know it works better than those UV wands (you need at least a few minutes of exposure).
It's also neat for a few other things that it's probably healthy to sterilize a bit more frequently (e.g. keys, phone, toothbrush heads).