Aside from appearance, different fretboard woods can also feel different to the fingers and some can have tendency to change colour more, or also wear more easily. I have ukes with rosewood and koa fretboards, and guitars with ebony, rosewood, walnut, and maple fretboards. On the guitars, the walnut is my favourite followed by ebony, while I love the koa fretboards of my Kamaka and KoAloha ukes. The major disadvantage of koa for fretboards is that it changes colour through contact with fingers, resulting in ugly wear patters over time.