YES it is much more difficult to sharpen ceramic blades due both to the extreme hardness AND the tendency of creating micro-chips on the edge. BUT it is not impossible to both repair small damage or chipping and also to polish and refine the edge to a very high level of sharpness.
The ideal tool of course is a powered diamond wheel with a minimum of 1,000 grit diamond. Even finer grits can be used to truly polish the edge. However, no one I know, except myself, is willing to purchase this kind of equipment.
Polishing the edge to get a higher degree of sharpness if it does become uniformly “dull” can be accomplished with very, very fine silicone carbide wet/dry sandpaper. This is not a fast process but does work and I have been including a small strip of 1500 grit paper with knives sold.
Instructions: Put paper on top of mouse pad or piece of cardboard to provide some compression & better control of knife angle. Now draw the blade away from the edge, (do not cut into the paper), with only a slight elevation of the back of the blade. 20 strokes on each side and then test the edge. You can use from 800 to 2,000 grit paper. Start coarse IF there is more than just polishing required.
Grinding out chips or damage to the edge or re-shaping the edge
Geometry is really only possible using a diamond hone. Powered would be really fast and then you can use a very fine grit. Using a small diamond hone (such as I am now offering for sale at $8.95) with a 600 grit does permit fairly quick removal of ceramic from the edge. I have found that 600 grit offers the best comprimise between rapid removal of material AND an acceptable working edge . Water is used as a lubricant and no more pressure than you would use with a steel blade on a hone. These diamond hones can be used on your steel knives but keep in mind that you will find the steel is VERY RAPIDLY removed from the edge! Actually a very useful tool to carry in your wallet!
Do not try to use any coarse diamond files or round/triangular rods. These will chip the edge as will most commonly used sharpening tools. A powered diamond abrasive with very fine grit 1,000 or finer, will produce a clean edge but may require further polishing to remove scratches and micro chips for the absolute best edge! I have found that a small 600 grit diamond coated steel plate does work very well to re-define an edge or remove chips etc. Actually a good working edge but further polishing with 1,000 & 1,500 grit SC paper will further improve this.