I have used the HF AD helmet for 5 years and had to replace one due to filling the lens full of sweat which it didn't like so for sure they aren't water proof. The one I have now for about 3 years has a weak battery that wont stay charged unless I hang it in the window so it gets full sunshine. It still works OK, just sometimes the AD doesn't work on the first rod strike for a milli- second. But after the first rod, the UV has built up enough charge to keep it working right. I have a new one in the closet that I should break out and start using but the old one still works well enough. You wont get flash burn to the eyes from it not darkening because the filter plate stops the UV rays it is just bright for a second.
Most flash burn happens (other than flashing from not having the hood down) not because of a faulty lens but a faulty gasket or one that was improperly positioned when the clear lens were replaced. It is a good habit to check the hood with a strong flashlight when changing the clear lens and also do a daily inspection. Just put on the hood, run the flashlight around the lens perimeter while looking for any stray light. IF ANY is showing, you will get flash burn if you use the hood without adjusting the gasket.
As for costs, the biggest difference that I see is the gadget settings for grinder, timers etc. A grinder setting is nice to have so you can use the hood for those grinder jobs rather than remove and put on a clear face shield. All of them now have multi-shade capability and they all work just as well as the next for UV protection IMO. Some of the hoods head gear will be much more comfortable than others and some designs look slick whether or not they are functional at repelling sparks is debatable. My take on welding hoods is that the users buys what he likes and what fits comfortably. Choose the hood that you like first then look at the price and then compare it to a cheaper one to see if you really need to spend the $300 to get it vs the $40 for a HF or Northern tool version.