Well, ok, I guess we can use that as a justification for anything then. For example, the “good reason not to” use E2E for ntfy could be “market analysis says the small and somewhat theoretical benefit isn’t worth the complexity.”
By the time you're trusting a hosting provider to properly do e2e for you... you've basically already lost the game. At any point they can update what's running and remove any/all protections you think you have.
So again - what is your threat model here? Because it sounds like you want "super convenient" and also "super secure" and those aren't two options you just check off - they're really more like diametrically opposed sides of the same slider.