Well, there are all sorts of pros and cons, but I guess most new materials get their development costs covered in the automotive and avionics areas - the boat market being smaller there's less scope to produce parts at a reasonable cost. Installations of engines in boats vary so widely that flexible hoses remain the only cost effective way to go.
Even if you made the exhaust system capable of handling the heat load, there are attendant fire risks (there's no air flow to cool the components) and the engine cooling still requires water flow. Once you're pumping seawater as a coolant you may as well use it to cool and silence the exhaust, alternatives are keel cooling or a heat exchanger in the same form as the radiator in your car and in the summer season it can get hot enough in the boat already.
It would be interesting if developments in this area lead to simpler and reliable systems. People advocate new technology for batteries although for weekend sailors such as myself they offer no advantages, costing maybe ten times that of old technology wet cells with a minimal increase in lifespan - one can afford to replace batteries several times over for the same cost. Similarly, silicone hoses are reckoned to be tougher than conventional materials, but similarly offer no cost benefit due to their price.
Rob.