Some VPN's IP addresses are flagged/blocked. Ive been finding that there are some sites that black list protonvpn IP addresses, where as they were fine with ExpressVPN.
There are things I liked with ExpressVPN, but some of the business decisions, were such that I've lost confidence in them, where as proton's privacy stance is something I appreciate. Also, and its probably the most important reason. I'm paying for their email service and I have the tier plan that allows custom domains, so I'm basically getting the VPN along with my email service. Why pay for another VPN service ¯\_(ツ)_/¯