The price for both hardware ($199) and subscription ($99/year) seems pretty reasonable. I could set something similar up myself, but then I would need to _maintain_ it, and doing email properly so Google doesn't mark everything as spam is non-trivial.
I don't like the idea of essentially opening up a bunch of ports to the internet. If you connect to their AWS endpoint on a bunch of ports (25, 80, 443, etc) that connection is transparently redirected to the device in your LAN over a point-to-point VPN. You can lock it down by putting the device on a VLAN and firewalling it off, of course.
Ultimately you're making a bet on Helm properly maintaining its software, security updates, etc, and more importantly being around longterm to do so. That's where I just stick with Google. Gmail (and GSuite or whatever they brand it as these days) isn't a service I'm at all concerned about Google kicking to the curb.
If you're serious about disconnecting from the cloud, though, this is a neat product and service. I hope it succeeds; if it's around a couple more years I'll switch.