They're bad for the game, because Ward is a hack.
Just look at his books. They're filled with needless padding, repetition, and overpowered nonsense for fluff and rules. The only way the fluff passes muster is to assume he is writing Imperial propaganda and it's exaggerated.
It's not they do everything, it's they do too much - Or too much the same. What is the operational difference between a Blood Angel and an Ultramarine? NONE. The BA book is arguably a better way to build a non-BA force than the Vanilla codex. Blood Angels has no valid reason to exist other than Marines = Money. Space Wolves and yes, GK, have the same problem. Meanwhile the one and only power armoured force not a Marine gets shafted.
They have weaknesses... specified ones that need special builds. The primary counter to such armies are armies of the same type.
Balance via Codex Creep has NEVER happened. There has been, in the history of 40k, one time where arguably things were balanced - the launch of 3rd edition up until a codex was released. Which codex this is varies, but Necrons normally wins the prize, ironically. Up to that point most forces were balanced enough, and Necrons, for their part, had balance in a way, but their main weaknesses were low numbers and phase out. The next codices added more and more power in the arms race.
Even looking at the current ones, there is creep. Marines are overpowered by IG, which in turn are overpowered by BA and GK, and arguably Space Wolves have their elements of OP. Balance isn't about levelling the playing field over time, it's about being level from day one.
Contradicting of established fluff is bad, when there's not a valid reason. Ward never gives a reason. He shoehorns and retcons, and makes it up. When you have a vast amount of background to work with, sudden changes without any real cause, cause chaos.
What he does with fluff has disturbing patterns. He often has the Avatar beaten handily. He kills Sisters of Battle. He throws in mary sue after mary sue.
And finally... He does ruin armies. Anything he touches turns terrible, and forces massive metagame changes. He incites rage amongst 40k players, and he has been the most visible cause of massive divide and anger in the veteran fanbase not caused by marketing moves.
Everything he writes hands over the Vanish/Doom glitch, the Javelin trick, and a whole heap of other barely hidden exploits and overpowered weapons for the savvy cheese merchant.
I won't link to it, but google for 1d4chan, Matt Ward. Some of it is RAEG but much of it is true, and explains why he's so hated.
What it all boils down to is, he is a classic case of Running the Asylum. He is a fanboy, pushing his agenda, making everyone else wrong. He tells us how awesome things are rather than showing us. He is basically saying that his way is right, and everything else is wrong. He is repetitive, predictable, and arrogant. His rules encourage the same traits amongst players, and he is the largest cause for 40k's rules degradation.
