Feature detection seems to be quite broken to me.

For instance Safari on iOS reports support for drag and drop events, but they aren't actually triggered. Another I encountered recently is the "accept" attribute on input file type. On iOS, you can't set specific file type, just mime type, but you have no way to detect that except by using the user agent. Then you have the buggy releases that make a feature seemingly supported, but is actually unusable in practice (looking at you IndexedDB on Safari).

This means using a library like Modernizr is almost mandatory. Also, what do you feature-test? Everything? Feature-testing something that has been available for 20 years sounds like a waste of time on the face of it, but the "alert" case has shown us that if you really want to do it correctly, you can't assume anything. I am not saying it is not possible to do it properly, but it is not that simple. For anything non-critical, I understand waiting for things to break then fixing them instead, it's way less efforts.

The fact that all three examples you listed are iOS Safari is giving me flashbacks to a previous job where we had way more time spent coming up with ugly hacks to make iOS Safari like something behave closer to a decent browser than we spent on all other areas of new feature development. And we also supported IE11, which was way easier to manage specifically because it didn't straight-up lie to feature-detection like iOS Safari did (and does).

It's probably a bit biased because I develop on Firefox/Chrome (very rare to find difference between the 2) and have to wait for reports/borrow an iOS device to test/debug. There might be similar issues with the aforementioned browsers, but the pain is less visible because it happens during development and I don't remember it. Also most of our users are using iOS devices, so reports for them are more likely. On the other my gut feeling is that iOS is more painful to develop for.

I'd argue it's very much on the browser vendors, because they create the strong expectation that everyone is supposed to always be using the latest version. They do this by bundling together feature updates and security updates. Web devs do the pragmatic thing and ignore feature detection, because they know users are constantly being told to update their browsers to keep themselves safe.