Yes the shortcoming is always with the weakest link.
But most of my mail is inbound, so I have control over that. Plus I created sieve filters to automatically tag and organize my mail, including deletion of unimportant stuff like newsletters. Top that with a personal domain and I have everything how I like it.
Anecdotally, in the many years I've been with Proton I've gotten maybe 2 or 3 spam emails, and they went to the spam folder.