Its really subtle. Maxing the sharpness as a picture setting tends to increase the likelihood of moireing of sharp hilights on the LCD. Think hilights in the eyes, hair, any fine detail in the scene. Since the resolving power of the sensor is finer than the LCD, when you max the sharpness, you can see moire on the LCD in the sharpest part of the Image. This isn't recorded, its a common visual artifact of displays when the sharpness is artificially cranked up.
A good test is to take a dark polished surface that has texture like a wooden table, and point the camer at a 45 deg angle opposite the light source. Move around until you can see the hilght rake across the table. Now take your M, set focus to manual, picture style to user where you crank the sharpness all the way. pointing the camera at the table, rack the focus.. as soon as you get close focus, you should se a moire pattern traveling up and down the focal plane. That represents the sharpest thing on the LCD. Move the focus ring until the area desired has the moire pattern on it.
In short, we're basically using the software int he camera to artificially sharpen the image to cause sharpening artifacts ont he display which just to happen to represent the sharpest part of the image.
In RAW this is fine but in JPG you will end up recording an overly sharpened image since your picture style was set to this overly sharpened filter. In the old days, when EVF had sharpen controls on them, I used to do this to help with focus. the difference here is that I'm applying this as a filter to my image. RAW is unaffected, but JPGS will have the sharpening baked in.
I use this mainly with lens babies were all you can do is focus manually. Another trick is to compose your shot, use the zoom to focus I was asking about above to scroll to the area of interest and magnify. Focus then fire the shot. Since you were already composed, and had the magnify rectangle move to your area of interest, you don't have to recompose.
I hope this helps!