Originally Posted by Valvefloat991

Such integrated manifolds are getting more common, but they introduce considerable complexity to the cooling system. That's because most heads are aluminum and, therefore, so are such integrated manifolds. Since the melting temperature of aluminum is lower than the temperature of full-throttle exhaust gases, these integrated manifolds must be designed with generous cooling passages in which the flow is regulated separately from the engine's main cooling flow. It's not uncommon for there to be three or four thermostats or valves controlling this flow.

As usual, there is no free lunch.