What is fastmail's sell other than their webmail interface? Seems very expensive to me. I pay $15 a year at mxroute for unlimited domains and users.
I personally don't really want to deal with cPanel, too. Not because cPanel isn't cool, but because I explicitly wanted to pay somebody to do the management for me so I didn't have to get too in-the-weeds. I'm certain I'd misconfigure something somewhere and I just don't want to deal with that.
EOD it seems mostly like personal preference and how much you want to tinker. Value for both seem close to the same.
I've also read mxroute's writeup on why they use cpanel, and apparently it just made sense to use it for user configuration, and that under the hood it's all a very unusual setup (I don't understand enough about email to be honest).
I wouldn't knock MXRoute, if you're happy with it, that's awesome. I'm a pretty happy FastMail user, personally, but I will say the account-based pricing has sniped me: I wanted a separate mail account on my domain for a very low usage, text only email handling piece of software... and it would double my bill to do that. I'd at least like to be able to have my different user accounts on different user tiers, pricing-wise, and currently that isn't available.
The billing is awful (they may have changed it and I may have not noticed) but I login every three months or so to manually renew a three month subscription, I don't think there's recurring monthly billing.
There is recurring monthly billing and I've never experienced any issues with their billing. Reference image: https://piqdd.com/fastmail.billing.screen.png
edit: looks like they have a deal at $60 for 2 years with 40GB, still unlimited domains and users, which to me is the main draw.