In the Global Top 10, Six are from NR, Two are from Monsters, One SC and One NG. I am pretty sure out of the 6 NR, at least 4 are with Foltest. I had already mentioned that Foltest should be considered as one of the strongest leaders in the game. But changing anything about him (reducing Mulligan or reducing the charges) will make him super weak. So, lets just assume that Foltest is a Tier 1 deck on par with Monsters. But I wouldn't say he is broken. Like RestlessDingo mentioned, he as a leader only gives +3 on board. There is a NR bronze (Drummer), Ves, and the Artifact who can give Zeal too.
I tried Foltest with a Tamerian Infantry (and 2 shields) and the Usual Suspects (GOD, Seltkirk, Eyck, Sasanthesis Blaze) and I beat Big Monsters almost every single time (at least 8 out of 10 times) at Rank 13. But I get may back kicked by many other decks. I am very new NR player in HC (but the most loyal NR player in W3; only played NR the entire time), and I find Foltest very interesting and powerful.