No seal is ever perfect, that's the concern. Plus helium, being monatomic, is able to jam into tinier holes than things like oxygen, nitrogen, or water vapour.
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 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.