The main advantage of a compound system is a wide powerband. My small turbo is rated at 800hp, so it will start making good power at around 400hp. Add 1 bar to the front of it and (assuming inlet temps remain the same) its good for 1600hp, add 2 bar and its good for 2400hp which is what the big turbo is rated at.
So I literally have a powerband that is capable of stretching out over 2000hp. Trying to do that with a big single or twins is impossible, 2x 800hp turbos will not start making good power at 400hp, that figure will be doubled so it becomes 800hp, then the total output is reduced to 1600hp which gives a powerband of 800hp and is twice as hard to get on boost.