The big things which many of his projects at GW are frequently criticized for are poor game balance/codex creep, and fluff mangling.
The Codex Creep part (for the non Warhammer/40k players, GW would release new "editions" with bare-bones rosters, reasonably well balanced against each other, then would release army "Codexes" one by one. Every Codex released was more powerful than anything which came before it, hoping that everybody would drop what they were doing and go out and buy a new army for several thousand dollars. This process continued until the game balance was so shot that the game was essentially unplayable, then a new "Edition" would be announced.
Whether it's his fault or not, Matt Ward was responsible for many of the worst offenders of Codex Creep. I hear from Fantasy players that his "Daemons of Chaos" codex was so game-breakingly bad that it started the player exodus from Warhammer Fantasy, culminating a few years later with Fantasy being killed off and turned into Age of Sigmar.
The Fluff Mangling part, while also possibly not entirely his fault, turned off a lot of old school gamers. Warhammer 40k had a rich, detailed setting, with many different factions, and players felt like their army had a distinct "personality". Kind of like Blood Bowl now! Matt Ward murdered the back stories of nearly every Space Marine chapter, essentially reducing them all to wannabe Ultramarines with different colored armor. He also consistently made special character stats which didn't really match their back story - supposed tactical geniuses (basically every special character is described as a tactical genius) decide that the best way to help their cause is on the front lines, swinging a power sword in melee. If he "reimagined" Griff Oberwald, he'd be described as a tactical genius, finesse star player, with MA6, ST6, AG3, AV10, Block, Mighty Blow, Leader, Pass Block.
Basically, the two best things which Blood Bowl has going for it are a mature, reasonably well balanced rule set, and a rich setting with a detailed history and a unique feel. So I thought I'd troll a bit by mentioning a game designer best known for screwing up game balance and twisting the fluff into unrecognizable pieces.

Oh, and Matt Ward has a hard-on for Space Marines like you wouldn't believe. If he did take up Blood Bowl, there'd be a Space Marine team before you know it. Probably MA7, ST4, AG4, AV10 linemen with Block, Guard, and Nerves of Steel, for 90k each.