The added strings add more latent energy, which can be good or bad. You'll need to build slightly heavier for a six string, and a little heavier than that for an 8. You also need to figure out how to fit 6-8 tuners on a head stock, without it getting hideously large. There's plenty room for four, but adding just two starts cramping things up and measurements become very critical. Other than that, they aren't much different from their four stringed brothers.