Where has he said he hates the stuff in IF? I've never seen that even implied anywhere. Just because he chose to make some changes to Iron Fist, doesn't mean he hates it, it just means he chose to take things in a specific direction and doesn't rule out him taking things in a different direction for Inhumans.
And doing a grounded take on one superhero, doesn't rule out the going for a more fantastic take on different ones later. Plenty of writers write in multiple genres, and this wouldn't be any different.
Hell, Joss Whedon wrote for Roseanne and Parenthood before he did Buffy, Carlton Cuse, who created Lost, The Strain, and Colony, also created Nash Bridges, Martial Law, and Bates Motel. Greg Berlanti worked on Dawson's Creek, Everwood, and Jack & Bobby before he created The Arrowverse shows.
Besides, doesn't Iron Fist already tend to be one of the more grounded series in the comics? I know we have the mystical stuff with K'un Lun, and the Iron Fist itself, but it's not like he's running around fighting aliens, or going to other planets or something.