Does PCB artwork affect operation and how is the effect validated?
Most artwork - with the exception of homemade boards - is done on the silkscreen layer, not in the copper layer, so should have no electrical effect. Artwork on the copper layer could increase parasitic capacitance in neighbouring traces.

Since it's rare for modern professionally produced boards to not have a separate silkscreen layer, I suspect the only people doing artwork in the copper layer are people producing their own boards - and they'll validate that it's not a problem simply by empirical testing.