Yeh, I agree. There are some nasty little WP pitfalls which can totally bite you on the ass if you're not used to it as an environment.
I guess I'd also like to chew into what "a website" is. I mean - I still maintain that for basically all "normal" content-rich websites you'd be hard pushed to find a better solution than WordPress. It's not just the rich plugin/theme/development ecosystem* but the fact that editing is so, so simple that makes this the case - so although many modern solutions for delivering websites focus lots on the front end, actually (in my sector / experience - working with very content rich, complex sites), it's "a nice editing experience" that is critical too.
At "transactional" level - ie as your "web thing" is less about pure content and more about "something app-like" then it becomes a different sort of problem. Saving state and logins and so on is absolutely in WP's space in theory, but you definitely run the risk of entering an arena where you're making it do things it really shouldn't be doing, and that can get really gnarly really quickly.
But - having said that (and back to the thread) - I've been pleasantly surprised that using WP as an actual (simple!) web app framework brings lots to the table, and not many people have thought about it in this way.
[* this is also a downside. We do "proper" WordPress development - fully bespoke "from scratch" themes, proper deployments etc - but many, many people just throw up a Divi site with 80 plugins and say they're a WordPress developer...]