They require a DM with sound judgement who knows when to say, "No," as well as players who are flexible. If you know that your players will whine and complain if you nerf something due to it overshadowing others, then you'll need to be more cautious and restrictive from the get go.
As players have more control over bonus feats, adding more books significantly ups the power level for melee types. Spellcasters, however, do not benefit as much from extra books due to the low number of spells known.
Also, you'll need players that either know what they're doing, or a DM who can help guide them during character creation. Otherwise it is very easy to end up with totally ineffective characters who are unable to fill their class roles.