When I was welding for a living, I found that using an auto-darkening helmet (large lens Miller) for GTAW caused my checks to sunburn by the end of the day (6-8 hours of welding), so I stopped using it. Another issue when doing the sort of acrobatic welding I was doing (laying on my back, hanging upside down, reaching around other parts, etc.), I got a few quick flashes when something I was welding around obscured the sensors as I moved my head and that can be disconcerting.

I don't use an auto-darkening helmet anymore, but only because I'm no longer welding full-time and the good ones were too expensive to buy on my own. Other than the above complaints, I found it far more convenient for the lens to darken than to flip the helmet down under those out-of-position conditions, and it may just be that I'm ultra-sensitive to whatever UV gets through.