First,the cast iron barrel engine is a different engine design than the AVL.
They may share some similar appearance, and there may be some parts compatibility, but the actual engine design of the AVL is more modern, and quite different in many respects.
The UCE is completely different from either the cast iron barrel engine or the AVL.
It's different in virtually every respect, except that it still retains 84mm x 90mm bore and stroke dimensions.
A carburetor works on engine demand and throttle position to allow air and fuel to be drawn into the engine.
A fuel injection system uses a computer and sensors to determine the throttle position and the engine demand, and squirts the correct amount of fuel into the inlet port to be correct for those conditions.
A fuel injection system is generally considered to be more precise, and can be tightly controlled, so it can be better for emission control, and economy, and also can be better for power. But, being controlled by a computer, it's not easy for the rider to make any real changes in the fuel mixture, if he wanted to change it himself.
The carburetor allows the user to make his own changes easily.