The immediate dangers are mostly in the context of usage, not the usage of itself. In a medical setting it is an extremely safe anaesthetic. However recreationally you have problems with people trying to run around and dance after taking it, there is a risk of tripping and falling. Same thing, when using a mask -- or just inhaling nitrous continually without breathing air -- there's a risk of hypoxia. That is what would kill your braincells. People have died both of these ways.

After that the big problem is sustained usage and addiction. You can develop a Vitamin B12 deficiency as the girl did in the article that can be very dangerous. When it was just individual canisters that you put into a whipped cream charger it was a lot harder and expensive to reach that point. But now with Galaxy Gas it is a lot easier to get to a more dangerous point. Before Galaxy Gas I've been around plenty of people that regularly enjoyed nitrous, and no one was getting anywhere near to what she was doing. As usual once something like this gets super commercialized it becomes more of a public health danger.

Not sure I would still call it "safe" but it is one of those things that if everyone was just better educated on the reality most of the risk and harm would be reduced.

https://pubmed.ncbi.nlm.nih.gov/26496821/