Acoustic Guitar Construction

I have not personally played an instrument that has a flat fretboard, but I do read here and there that a number of the top-flight fingerstyle artists insist on it. If I'm not mistaken, most classical guitars have the flat board also.

What are the advantages/disadvantages of the unradiused board?

Classicals are definitely typically a flat fingerboard. On my jumbo, the first I built, I radiused by hand, so couldn't tell you what it was, but it was slight. Since then, I have been using a 12' radius block. Not as pleased, the fingerboards aren't as comfortable to play. I think (and could definitely be wrong) the radiused fingerboard allows you to use a narrower neck, yet still space the strings without cramping them. Some find it easier to bar on a radiused board because you can curl your finger over it, which is a stronger position. I think the next few I build I will go back to hand radiusing them, or getting a larger radius sanding block. I find when strumming, if the saddle follows the radius, that a heavy radius emphasises the middle strings, and you don't get as strong an attack of the bass, and can just about miss the treble (at least the way I strum). I prefer rich bass and crystal trebles, and not a lot of midrange, so anything that detracts from that bothers me, and this is the effect I get on my mango guitar. I find that a very slight radius is comfortable, and a larger one causes me hand fatigue for some reason.

Martin 16"

Gibson 12"

I constructed a prototype with a flat fingerboard per Sloane's instructions -- the guitar was for my brother, after the accolades wore off, truth came out that he hated the flat board so I replaced it the following summer!

I plan on having a fretboard radius of 24", on my next classical guitar. The are a few good classical guitar builder that are starting to put a radius on th fretboard. From what I have read it is more ergonomical.