The reaction rate of any etching process is limited by local current densities, access of the reactants into the reaction area and clearance of the reaction products away from the reaction area. Since board etching is essentially a planar or two dimensional process this places further limits on etching performance with reactant delivery and reaction products actively interfering with each other for access to the surface.
While always present in processes, where the problem arises is in the differential etch rates across the board. This can cause thin traces to be etched at a different rate than wider traces. For example, etching a relief from around a fine trace within a background of a ground plane is very different in loading than etching a thin trace with no background ground plane.
This can be corrected for by ensuring that in the design the pattern density remains fairly constant per unit area across the board. Thieving is one way to do this. Some manufacturers will actually place sacrificial elements within the tanks and along side the board to ensure proper yield of different line thicknesses.
Mixing and agitation of the tanks during etch will also help mitigate the differential etch issues.