As with any material that comes from nature, cork can have inconsistencies inherent in it that can create problems down the road, either during assembly or during use. Cork is more expensive than foam. For some, synthetics are the materials that they'd rather have their handles made of. Older materials used in synthetic handles had a bad habit of hardening or breaking down after lots of use, or UV exposure. The newer materials are supposed to be much better than in the past.
Synthetics are generally very homogenous throughout.
Since they are man made materials, quality control during manufacturing insures the quality of the final product. The latest synthetic materials are tougher than cork, made of closed cell material that won't absorb water, and is UV resistant. This characteristic makes it very nice when it comes to assembling and gluing your handle. If the blank is larger in diameter than the I.D. of the handle piece your are mounting, it will expand to fit the blank. There are limits of course, but fitting cork requires more precise preparation than synthetics.