A pleasure after a 15 mile hike chasing elk but a down side is most folks can't shoot them as accurate due to less mass. A one piece is quicker pointing if you like shooting running or flying targets. They can be built as a 2 piece TD using a slip sleeve. Using the slip sleeves, when assembled it looks just like a one piece if you don't like the looks of the 3 piece. For the most part, the wood used in the riser is personal preference.
Most bows do benefit using a riser wood with good mass to it.
Exotic woods can be very pretty and can have exceptional mass but strength and durability can become an issue. Take a look at my Wood Examples page for more detail on many of the woods available. The stand out in the crowd from a performance stand point is Tonkin Cane. Tonkin Cane is bamboo that comes from the Tonkin Gulf region of S.E. Asia. It is considered the best bamboo because the climate of the region makes it grow with stronger fibers than bamboo found in other areas.