This is really a bad title to our Pro/Cons page, but in principle the takeaway here should be - email is not perfect and we chose some compromises (unless you use hey.com, they "fixed" email ;). That is why you are reading this as a Con to our service, not a Pro.
Indeed, for us, encryption at rest is only meaningful if we cannot access the mails either. However, that is nowhere on the horizon for email. There is a technical way to achieve that, but at a great cost of usability.
Encrypting the disks themselves is trivial and is on our roadmap, but it is nowhere as important as some providers tend to boast. That is why we say "quackery." It is just a "nice-to-have", but in every-day security it matters just as much keeping your car keys in a sealed jar all the time, carrying the jar in your pocket =)
Our data is stripped among multiple disks in RAID10 (obviously), that by itself ensures very little importance to a single disk. Not to mention that if a disk is dead, one would have to find it, identify it belongs to specific user and recover it. This is more likely:
https://xkcd.com/538/
Processes inside of the data center are way more important than an extra layer of encryption for a hollywood heist.
In our so far experience, the biggest threat to users' data are the users themselves. Forgetting passwords, accidentally deleting mails, running malware, choosing weak passwords...
It is interesting that we have never seen this issue raised for Outlook, Gmail, Zoho, Yahoo, AOL and other large providers, who never did or will encrypt at rest.
With hosted services there has to be a chain of trust. We trust our providers (ISP, data centers etc), and our users trust us as a provider.
Furthermore email is generally being observed wrongly today [IMHO]. Email message is a equivalent of an open envelope, a postcard. The only way to security is end-to-end encryption. This way, the need to trust the provider is removed. We could say openly we do have encryption at rest, but that claim cannot be proven by anyone. Same way as Whatsapp says they are E2EE but being a closed protocol, we still have to trust Whatsapp they are telling the truth.
As a provider you have to choose what is worth implementing and what not, what realistically benefits users and what not. Military grade security is silly for email in our opinion. If that is requirement, one should probably not be using email at all. Use Whatsapp :D
Jokes aside, we just try to keep a sane approach to security and not take absolutist stand, all or nothing. It is always a compromise between usability for the user, maintenability for the sys admin and cost. We are not on the expensive side meaning we have to take sane compromises.
If you are willing to pay a premium for absolute security, we can of course do that for you, but we know no one will be willing to pay, and those that do, we probably do not want to have such users, based on our experience from @protonmail users using Migadu. There is tutanota for that purpose btw.
Email was not meant to do many of the things we today try to use it for. It is much more complex than majority will even know and appreciate unless they become providers themselves.