Frozen tissues can be very challenging samples to stain. Timing is so important and teams are consistently asked to shorten stain times, allowing the pathologist to respond quickly to the surgical staff. That said, it can be difficult to find the balance between quality staining and speed.
One of the biggest artifacts I have experienced is uneven staining unrelated to tissue sectioning. The number one reason is rushing the fixation and rinsing of the slide prior to staining. Remember that the media used to support frozen tissues during sectioning is water soluble and MUST be removed from the sample, not unlike removing paraffin from routine samples before staining. The media can prevent even dye infiltration just like paraffin wax does. And, not unlike wax, you will probably not see any residual media left on the slide at the end of staining, because steps during staining will remove the media along the way.