Velvet Drives work fine when they are mated to an engine that is suited for them, like the Ford Lehman 120. They may not be so reliable when mated to a higher-powered engine. They have been used on a billion boats starting in the 1960s so have a pretty good record of reliability. Like anything else, they have to be operated and maintained correctly to get a long life out of them. Probably the hardest thing on them is impropper shifting technique. There are several versions of the BW transmission--- the drop-center versions seem to be the ones that are the most trouble-prone.
As to parts and maintenance, it may depend on where you are located. Here in the Puget Sound region of the PNW there are a number of marine gear and diesel shops that work on Velvet Drives and so far as I'm aware, parts are not an issue. This may not be the case in other parts of the country.