This is my first time hearing about it. I read through a basic description and it sounds interesting in theory. What makes it rubbish?

as someone who has shipped a few projects made mostly in QML, it's clearly not rubbish

I sometimes wish the effort that went behind QML went somewhere else. That to be said, clearly other folks are finding benefits in it, just not for the app I have to support (yet).

The statement is overly harsh, but essentially correct.

QML simply lacks a lot of features, and C++ hooks are often required to make it work well.

Oddly, you need to launch a QML app using at least a little bit of C++ no matter what. That's not a problem for mot pro devs who can hack a little bit of C++, but it's actually problematic for most others. Even 10 lines of C++ is too much for most devs who don't know it. It's like having to speak 10 lines of Russian. The docs are often tricky and ambiguous about how to start and launch apps as well.

That is like... The two usual ways to use it. Although I recommend using it with C++ for nontrivial applications.