Matching colors.
I can't for the life of me match white-dyed gear with AF or other unique equipment. Snow white dye stands out too much as eggshell compared to PLD AF, Foestriker white etc.
Another thing that bothers me are the gloss textures. Examples would be Cobalt Cuirass, PLD AF and Toxotes Tunic. It's pretty hard to match those when other equipment can't be dyed to metallic like those.