I'll keep it short, but provide links for further reading. First off, I don't think proton is "bad", or anyone is trash-talking them legitimately. Both their jurisdiction (Switzerland), dedication to FOSS and good audit results make them a solid choice.

Diversification is my main reason why I'd never go all-in on Proton. Over the years, they've grown an ever more complete suite of apps (e-mail, calendar, VPN, password manager, docs, AI) that does share some similarity with the Google app suite in that you're essentially putting all eggs in one company's basket - and a lot of trust with that. If Proton got hacked, got subverted by a three-letter agency, or just got taken advantage of by a rogue employee, the effects would be much more impactful than if it had just been your e-mail provider, just your VPN service, just your...

Also, Proton's official social media account echoing CEO Andy Yen's private statements on U.S. politics haven't helped convince people that the company is, indeed, politically neutral and solely focused on privacy.