Downside: the adapter I bought for my Light & Motion housing has proved to be fragile. It cracked in half the first time I used it; thankfully my little lanyard saved it from the depths. I suspect the printing process that apparently was used to make the holder resulted in a fairly weak piece of plastic. If there's a way to get or make a stronger holder, I'd suggest doing so.
Quite a while back a very similar "sunburn" happened to my viewfinder, only this was in the Galapagos, and it was a Sony PD-150 in an Amphibico housing. Expensive fix.
I thought at the time it was during a short ride when the housing was on the floor of the Zodiac, going to a dive site. The housing had a sort of magnifier on the viewfinder which I still think made the burn happen much quicker.
After the repair I searched out and put a small rubber cup over the eyepiece whenever the thing was topside. Never repeated the problem.
Amphibico, alas, had "never heard of the problem" and didn't have any cover available for the viewfinder.
I think it is a good precaution to take to cover the eyepiece on any rig.