Well...there is a great deal of difficulty apparently in building a composite epoxy hull and there have been admitted failures. If you read the "Tartan Buyers Pls Note" thread under Buying a boat forums you will see several owners with big time problems and a series of interview links with designer TimJackett that detail some of the issues that had to be worked out...sometimes in production...with the construction techniques and bonding issues. Both Tartan and C&C boats are built using the same technology. I believe the thermal distortion issues around dark hulls Dawg mentions can be solved with outer mat layers of vinylester. The Jackett interviews cover this issue and I think you will find them interesting. Just remember they are the company's views and there are at least two lawsuits currently filed and a number of folks who disagree quite vehemently with Jackett's dismissiveness of some of the issues.
I can't think of any disadvantages to lead keels.
One additional disadvantage of carbon fiber masts is that many insurance companies will insure the boat but not the mast. A guy here a month or two ago lost his due to a lightning strike and insurance excluded it and he had a big bill.
No longer posting. Reach me by PM!