I have had plumbers install them before, and never had an issue. Well-respected builders, who build many homes, advertise that they are using Viega Manabloc, and consider it a positive piece of promotion.
But a new plumbing contractor is on board, and he says they are no good and does not recommend using them. When pressed as to why, things get vague and he cannot say exactly why. Something about plastic being no good. I say, well, we're using plastic PEX supply pipe, and the whole darned DWV setup is PVC. The line in from the well is plastic. The whole inside of his pickup truck is plastic.
He wants to use the Pro-Pex one-inch copper manifold with brass ball valves at three-inch centers. Cost is well more than twice that for the Manablocs, and considerably more space is needed.
Why not the Manabloc? Certainly hundreds of thousands of these must be in use?
The Manabloc had some problems a few years ago with there plastic ferrule connection on the Manabloc itself. They fixed the the problem. The nuts were losing themselves a few months after being installed. Most plumbers don't like the manabloc because of the labor it takes to install the system, plus it takes atleast twice as much piping. The system is fine.
Will Rogers Plumbing