I've asked people who used the first gen He drives because it's an obvious worry, and the consensus seems to be that they're at least as reliable as conventional drives. MTBF doesn't seem unusually bad, and can even be better than some trad designs.
This is different than the typical failure scenario though, isn't it?
You might have a few drives with bad seals that fail early. But say the majority are sealed properly, but the helium still leaks out at x small rate over time, you might have almost all the drives suddenly fail at around the same time after say 6 years or whatever.
I don't think you can really know if they're truly reliable in that sense until you can take one that's been sitting one in a drawer for 10 years and plug it in.
A company like Backblaze is bound to find out before the consumers do but I have a feeling it'll be fine. Keeping gas inside of something isn't much harder than keeping it sealed enough that dust stays out.
A high-altitude environment might put more pressure on the seals than other places, that could shorten the life-span, but other than that it shouldn't be a huge deal. Most drives have a commercial life-span of no more than 4-6 years anyway. After that you're on borrowed time.
