How is Monte Cook controversial? He is most popular and appreciated PnP designer after Gygax died.
Most popular PnP designer after Gygax? There are shitloads of people in the D&D community who think that he's an utter dumbfuck. I'm not one of them, just for the record, but neither do I think he's the fucking Jesus of RPG system design.
He's mainly controversial because in his columns and blog entries and other stuff he wrote while he was still involved in D&D (and for a while after) he consistently displayed almost complete ignorance of how some of his Ivory Tower design ideas affected real world P&P D&D gameplay. And let's not forget that the game he designed (D&D 3E) was so fucked up that there was D&D 3.5E, which was basically a bugfixed version. And then there is Pathfinder, which is basically a bugfixed version of D&D 3.5E, which was still a fucking mess.
Probably his lowest point was when he tried to justify D&D3E's terrible character developement and its clusterfuck of feat chains, feat taxes, purposefully misleading feat descriptions and other wilfully obscure nonsense and unnecessary complexity with "rules mastery", i.e. the retarded idea that players who really know the rules well should be rewarded by being able to build better characters. And, according to Cooke, he got this great design idea form looking at what people enjoyed about Magic the Gathering, i.e. building clever decks and shit. Then he tried to apply this concept to D&D character development. He essentially admitted to putting bad choices into the system and concealing them to create this "system mastery" element.
Now, this concept is actually
awesome for a CRPG system. Let the idiots play on easy and not put any thought into character development but make the system deep and complex so that the Sensukis and all the other autistic munchkins can treat it like a fucking math problem and come up with power builds that lets them solo the game on nightmare difficulty.
The problem was that D&D3E wasn't a CRPG system, it was a PnP system and in PnP, this concept turned out to be a fucking desaster. The sprawling complexity turned out to be utterly unmanageable so at some point after D&D 3.5E hit the bookshelves they simply said "fuck it" and just added to the complex mess by shitting out even more splat books... I mean, they didn't even TRY anymore because there was no point to it.
The "system mastery" element was even worse. Why on earth did Monte think this was a good idea in a non-competitive game like D&D? All he achieved was that the more casual DnD players (actually a majority of PnP players), who didn't spreadsheet the characters in advance, ended up with shit characters and the munchkins of the group ended up with optimized superheroes. What was the benefit here for your average D&D PnP group which probably consists of mainly casuals and maybe 1 or 2 munchkins?
This whole concept is so completely baffling in a PnP context that it's probably justified to call Monte Cook a dumbfuck based on this idea alone. I really can't shake the feeling that Numenera turned out to be so incredibly lightweigth because people have been giving him shit for years for being a guy who would do complexity just for the sake of complexity. There were a lot of people who were not particularly unhappy when Cook left D&D 5E developement due to "differences of opinion" for fear of him turning 5E into another overly complex clusterfuck.