Why you shouldn't use Material Design text fields. They are a bit of a pet peeve for me, especially with Material becoming the new Bootstrap on the web. It just feels clunky.
The whole fact that the labels slide around seems like animation for animation's sake, without a lot of regard for usability. I know that it feels nice to cook up new paradigms—I've done it myself back in the day—, but from my current point of view I would be against it. If most field labels you see are beside the field and don't change position, then go with the flow instead of inventing your own way.
The app I'm working on will take inspiration from Material, especially regarding the spatiality of the layout and interaction design, but not the visuals per se.
Google themselves seem to have been striving away from pure Material in their recent app updates. Apart from using the card metaphor, I don't think Google Maps on Android is really that Material-esque, for instance.
Another worthwhile link from the same article on the same topic.
(On this topic, another phenomenon I'm not a fan of, though perhaps it is happening for the greater good: replacing the navigation drawer with a bottom tab bar.)