> most of my experience using similar features in other products (Intellisense in Visual Studio, whatever the equivalent broken thing in Xcode is called) is that it really was never that useful?

No, they are not useful

I have been using IDE for sometimes, none of the features helped me to solve very complex logic in the codebase. I have to debug, test and trace the errors manually

And those who often refactor their code (with their IDE), I don't know if they are designing the architecture correctly. Once in a while is fine, but if they are doing too much of that, yea probably they are messing up the codebase. I don't really want to work with somebody like this, imagine that you have to solve the errors that they produce and IDE doesn't actually help at all