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?)
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.
It's also dangerous to say because you don't understand or see the value in something, that there possibly can't be any.
Today's home connectivity + LTE fail over is reasonable to rely on. One could put a vps proxy in front of it if you really wanted.
Running a home appliance is not out of the question or unreasonable. I have a Mac mini server that is coming on 8 years of age and zero issues.
Today, the combination of Ubuntu, docker, and ready to go setups make it super easy. Offsite backups are not an issue anymore. Running owncloud is good for files locally, but email is worth it in some cases.
We own a lot of appliances at home that have a lifecycle to maintain already.
The reality is the above issues have a much lower chance of happening than 10-15 years ago.
Hardware is far more reliable and than it was, my 15 year old servers pulled from my data center were still working when I virtualized.
A discovery I made was owning a PDU (like an APC Masterswitch) cleans the electricity so much enough that attached equipment don't seem to fail. I ran my own email server in a datacentre for clients for a long time because it was the norm.
With Home Automation adoption increasing I suspect a home appliance of some kind will become a reality anyways. If personal data became a feature of it, that would be useful.