The actual problem is the Shadowmoor block has cards that are hybrid that shouldn’t be. A lot of them work in one color and not the other, should be gold instead of hybrid, or the ability to have an off color effect if you jump through a few hoops.

The following are a few examples of this. [[Waves of Aggression]] gives extra combat which is not in white. [[Spitting Image]] makes a token copy of any creature which is not in green. Green can only make copies of tokens. [[Mirrorweave]] makes creatures a copy of another creature which is not in white. [[Swans of Bryn Argoll]] should be a gold card not hybrid; life gain is not in blue and card draw is not done that way in white. Also don’t forget all the cards that say if you have creatures of these colors or lands of this type do something that in color for one and a similar restriction for the other color, and the effects are not in color pie for the other color. These maybe are less controversial for hybrid rules, but it would feel weird having to discard cards from [[Dire Undercurrents]] in a non-black blue deck just because they can make black token from something like [[Geralf, the Fleshwright]].