You might like Proton Mail because of quality of service, or privacy policy, or not hosted in the US, or other reasons like that. Fine.
But AFAICT, there is not a compelling technical argument for their service •in realistic practice• being significantly more secure or resilient to server-side surveillance than any other credible email provider.
Again, if somebody with deeper knowledge of Proton Mail’s technical guts has better info, please let me know.
/end