Protonmail is another concept, their concept is build on how to make it super easy for customers. That's it. Even a client can be a problem, that has nothing to do with an web app etc. You also can't never be sure what is really running on an infrastructure. You can never be sure as customer when you buy a email installation service from someone that the someone doesn't install shitty. 
As you see we have the problem everywhere and there is absolutely nothing special with Proton.