Lack of attention to detail.
We hired them to design and do our landscaping - side and front yards. While the design was nice in principle, several aspects were not well thought out. Plants were often too crowded in some places and inappropriate plants were planted in others. For example he placed a couple of fruit trees and blueberry bushes in the shadiest part of the garden, not surprisingly, three years in they all look sickly. Little attention was paid to the topography of the site, so many of the lawn areas are sloped in strange ways, creating massive runoffs during storms. We also hired Joe to manage the work site and many of these things could have been corrected on site but weren't due to the fact that he was not around much and not properly communicating with the landscaper. Several plants died, and while he offered to replace them, he hardly returns calls and does not do work he agrees to do unless you call him every day. We eventually cut our losses and found a much more responsive landscaper who has been slowly correcting their mistakes and lack of attention to detail.