A downside to having persistent storage that is not mentioned is that a server could potentially be compromised in a way that survives restarts. No amount of disk encryption will protect you from that.
If the machine has no persistent storage and boots from a medium that is provably read-only (i.e. a DVD or a netboot image), it's always going back to a known state every time it comes back up.
Also "keys off site" is nowhere near as safe as keys don't exist. (can't exist, it's not a matter of a key when ram loses power)
Also "we don't log" is quite far from "it doesn't matter even if we accidentally logged, because the ram has gone poof".
The ProtonVPN setup is probably quite good and more than good enough, but it is disingenuous to claim that it is actually equivalent.
Encryption and policy are not the same or just as good as does-not-exist.
What could possibly be true, and so what they could possibly say honestly is that it's close, and the operational gain is worth the security loss.
I know how my mom can fail to see the absolutly binary 180 degree night and day difference between something that is difficult and something that is impossible, and see them both as being close to each other way to one side of a slider, but I don't know how anyone who programs or administers a computer fails to see those as actually being at opposite ends of a slider with only 2 settings, possible and not-possible.
If the machine has no persistent storage and boots from a medium that is provably read-only (i.e. a DVD or a netboot image), it's always going back to a known state every time it comes back up.