I understand where they're coming from--they have so many shitty designed apps on their play store that they probably felt the need to give guidance--but there are still many of their guidelines that are too subjective and i disagree with.
Basically they too have no idea what they're doing. One good example is their bottom navigation bar. They used to "recommend" that you shouldn't use those but use the top header, but found out that it's much more convenient for users to have the buttons at the bottom, so they changed their mind and added the bottom navigation bar. Knowing this, I just use it as a guideline but never think of it as an absolute rule. It's bad for building cross-platform apps too.
As a developer (clear distinction from designer) my favorite thing about mobile development is the UI components. I can create something much more compelling with MD with much less effort.
Other than that, yeah, I like it in general.