What if an autodarkening helmet fails?
Well, first, it's not going to be the most comfortable thing to be exposed to live arc with the naked eye, but it is doubtful that happens.
The automatic shutter controls brightness, not the ultraviolet. Much of the UV screening for an autodarkening helmet is from the glass itself. When it darkens, that's to help a welder see better as much as it is to protect his eyes. Most helmets will fall dark when broken. The light filter offers some protection, as well. A quick glimpse of the arc is less trouble than a bare sight of the arc. Auto darkening filters are mechanically a bit stronger than the plain glass ones.
Ultimately, any autodarkening lens that meets the ANSI Z87.1-1989 standard--and most do--is the equivalent to a #14 filter for UV and Infrared even when it is off so you get dazzled but nothing more.