I had a great experience with the service so far, no hoops to jump through, no obfuscated pricing, no personal data.
Their VPN and Server options are very pricey, but the domain prices are great.
I may have misunderstood your question. If you are looking specifically for a top level domain, I think .org is a decent one, as it is hosted by a non profit organization (even though there were some sellout debates in 2020)
The .onion means it's an onion service. This requires a Tor connection both for the host and the users. And frankly, hosting an onion service is not exactly easy to do it right, due to additional security considerations. Many self hosting guides assume a typical residential connection or a rented VPS without an onion service.
Yeah but this way you can only communicate with other Tor users. Also the mail server can prolly easily leak your actual IP if you don't harden it well.