I have a set of generic Chinese 50mm wheels that I've ridden for years. They were sold under the brand name Karbona but were bog-standard Gigantex rims, Joytech hubs, and Sapim spokes. Broke a spoke once and that was a pain in the butt because of the internal nipples and the fact that they are tubulars, which meant having to tear off a perfectly good tire.

But that's the thing for me: a cheap carbon tubular rim doesn't have the same potential for failure that a cheap carbon rim has. If the brake surface on a tubular delaminates, you're not going to have the same chance for a blowout and possible catastrophic failure that you could with a clincher.