Application - you can estimate the quantities of oil gas and water in a single pipe.
Accuracy - varies widely depending on the metering technique used, the ratio of oil / water / gas and the vendor claims. You often find that a certain meter or vendor is good for one ratio of components, but once you vary that by 30-40% the accuracy versus a separator gets more variable
Personally I don't think it gets much better than 10% and often a whole heap less in the field as the conditions vary so much, including how much slugging is going on.
An emulsified liquid phase would be difficult I suspect or need some sort of radioactive measurement device.
Best to do a trawl of the relevant vendor product data sheets and see how it matches your particular fluid.