I'll defer to those with alot more experience than I, but as I understand it, the difficulty to roll cast with a "faster" rod is that there is no space nor weight to flex, or load, the rod. You are more or less creating a forward cast with no back cast, using the weight of the line (and the current of the stream) to create a flex in the rod, giving the line energy when moved in the opposite direction.

With a rod that flexes more, i.e. slower, you can create more energy with less space/weight. With a rod that flexes less, i.e. faster, the rod doesnt bend or load the line as easy, making it hard to generate any action when you bring the rod forward to create a cast.

Similarly, fast rods have a more difficult time making short casts for the same reason. You arent generating enough flex on them to get energy out. Conversly, they throw alot of line a great distance when you do get them loaded, for the obvious reason they are stiffer and create more energy under greater flex.

OK<that was my first attempt to answer something technical. Tear it up boys.