Not sure if you have spent any time around data centres, but they have to deal with power outages just the same. My power doesn't go out often, how about you?
Backup batteries (UPS') are trivially cheap as well, especially for equipment that sips electricity. Everyone should have one on their internet connection and wifi anyways. A $200 ups can give a few hours of power.
Your offsite worries could be alleivated by mirroring 2 devices, or mirror one to a cloud. Realistically, incoming email is queued for redelivery from 24 to 48 hours until it can be delivered. Beyond that, outgoing email can be sent thru a third party temporarily (migadu, etc). Next question?
Migrating email isn't hard. Most services have import feature. I suspect you may not have done this before.
The challenge with email is that once you give out any mail address to people, you are on the hook to ensure that it's a functional address. Not something you can just try for 6 months and then easily move on from. And if you decide to move on from this service, will the average person be able to easily migrate that custom domain to a different email provider? (Yes, technically this is possible, but can normal users do it easily? Is it part of the service that Helm provides?)