Given the target market, I don't find Sharepoint that bad given the alternative offerings on the enterprise space.

All of them have different levels of pain.

Well.

Someone has never seen the advanced permissions tab...

There is not "eh, they're all kinda bad" left in your head after you try to understand that mess. And forget actually being able to validate your permissions, it's basically impossible without some actually licensed fake account that you can use for testing.

Also, in what world is "denying someone access to a folder" the same as making the folder invisible? What went through the head of the designer?

If you use it in a way that you don't know it exists (e.g., Teams as the UI, open files directly into Office apps, collaborate with others in real time both in Teams and in the documents themselves, where Teams channels are SharePoint sites but you don't know that), then Sharepoint is pretty slick. Just, whatever you do, don't poke it. Let the Teams UI manage it. If things get wonky, recreate Teams and Channels, Microsoft iterates the defaults and integration wiring, but doesn't reach back and rewire existing things.

It's the only way I'm aware of to have a modern tech like office productivity suite that's genuinely compliant with security and compliance regulations applying to the most heavily regulated industries. Google's offering is not that. You can get there with mashups of other tools if you start with a compliant (but clunky) groupware live discussion offering and identity aware role based access to a versioned collaborative document store.

Most firms are not required to have this level of regulated compliance, so most firms don't have to put up with the downsides. If you do, it is the least worst.