So I haven't gotten my hands on a copy of the new Year 12 MIF textbook yet. But at first glance from the Year 11 MIF, I feel like most of the questions have stayed on the same difficulty.
The textbook isn't really garbage in my honest opinion. At the very least, it sets the foundation. It introduces the techniques you require, and prepares you for further practice involving it.
But yes, the textbook tends to stick to a foundation. If you're the type of student that loves fast-tracking, you may want to consider alternate options. From what I've seen, the Fitzpatrick textbooks have a very healthy span of easy to hard questions. Same goes for the new Cambridge textbook.
And meanwhile, Terry Lee's textbook definitely jumps to the deeper end more quickly.
I do think that there is a gap between MIF and the harder questions in the exam. By itself, I feel like it would be inadequate. That being said, mixing things up with past papers can work miracles. Back when I did MX1 in Year 11, I literally just sucked up MIF and spammed past papers to help boost up my mark instead. (In saying that, if you want a more consistent option, having a back-up textbook with typically more challenging questions is a good idea.)