Sometimes custom skins and skins from the site make it more difficult to play by having a cursor which tends to blend in with backgrounds, and having hit scores (50, 100, 300, etc.) which are so big that they interfere with visibility, not to mention that some of them are kinda flashy. The default doesn't cause any problems for me so I just stick with that.
Anyway, my only issue with a skin is its hitsound and transparency for smooth play. Like, I get comfortable hitting those circles with a nc hitsound and get comfortable reading the map with transoarent sliders. Somethin like those.
I use skins that don't take to much focus like Luminous (v2) (link, https://osuskins.info/skin?s=2474). IT doesn't have any numbers in the circles and that does have pro's and con's. The pro's is as i said before it doesn't take much focus and that when ever you get a miss, 50, 100 or a 300 it isn't distracting and in your face but the con's are that I really like jump maps but sometimes I become confused with the order of the circles. If i would change the default skin I would make the colors less vivid and the miss's, 50's, 100's and 300's less distracting.