That's been my impression as they 4e monsters. They have a lot of good ideas, but seem overdone in some cases. I'm also getting a bit tired of Matt Colville's attitude about his game and everyone else. I get you have to market your game, especially given the success of his Kickstarter, but it stills feels a bit much. I'd prefer he concentrate on his game and not worry about other games/Flee! Mortals! Is a gorgeous book with interesting monster design.
My own experiences using them has been mixed. A lot of the monsters are made “interesting” by overloading them with mechanics. As a DM who likes to keep up story beats during combat, this has been quite taxing on me. The minions are a mixed bag, sometimes hard to use right. The Fire Giant minions were a huge letdown in my game, as these weak giants jammed up the dungeon and had a hard time doing anything worthwhile. In the same dungeon I used the other fire giants, where a support fire giant made the combat last longer by healing away a lot of the damage dealt by the party. It wasn’t exciting or interesting.
Solo monsters are great. You can feel they are tested well against level appropriate characters. You can trust them to challenge the players right.
I also used the Abominations villain party at the Abbey in Curse of Strahd, they fit there quite well. I hated that one of the monsters had a teleporting grenade. That took me and the players out of the fantasy. MCDM seems to prefer mechanics over ease of use and verisimilitude.
The monsters feel a lot more video game-y as they have tons of complex mechanics just to be “units” in a war game.
If you want more powerful monsters because of more mechanics and great art, it is a good buy.
If you prefer elegant design and more fitting monster abilities, you can skip this one.
My personal preference is to use a mix of simple and FM monsters.