Wrinkling can occur. Fiber reorientation may occur, affecting both the processing and performance of the part. The mat can deform around a complex tool shape via fiber reorientation and mat stretching and thinning. This works pretty well, but the same mechanism doesn't work for a fabric. In a fabric, the fabric won't stretch in the fiber direction, so the only way to deform around the tool shape is shear.
Problems can arise if precise fiber orientation is needed.
Fiber reorientation may affect not only the performance of the final part, but also processing due to changes in permeability. Fiber volume fraction may be too low in places, leading to resin richness or cracking due to residual stresses. In other places, the fiber volume fraction may be too high, preventing proper wet-out of the preform and causing voids in the final part.