Because of their skill, no one else in the church can keep up with them. I personally find this a distraction because I very much prefer to learn songs by following the score. Worship teams, if not carefully maintained and supported, can become a vehicle by which the musical education of the congregation is inhibited - or prohibited. This is because a rote learning approach to acquiring songs produces quick results, but ultimately doesn't provide a technical musical education. It's a lot like giving someone a fish as opposed to teaching him how to fish.
Worship teams can, by their choice of repertoire, cheapen the doctrinal stance of a church.
Worship teams, by their sheer expertise, can shoot themselves in the foot. Having a worship team, as far as I can tell, is okay. However, since the best musicians in the church are frequently up front, then it means that there's less time to be involved with those who are outside the group. The ultimate result could be a day when everyone on the team retires and no one in the congregation is equipped to take their places.