Its hard to freeze pipe's in large installations. It takes ages for them to cool down and the water in the pipes has got to be still.
You also need a point at which you can check it is frozen. You can use pipe thermostats of course, one either side of the ice plug, but somehow its not the same as checking no water is passing through a valved open end.
You also, as has been said, need a good CO2 gas supply and large pipe cuffs.
But then draining down may be a long job as well.
If for instance your only going to require it frozen for a short period, I would look at time and materials costs for freezing as against the same for draining down.
At least with a drain down, your not on pins all the time wondering will it stay frozen. But then freezing means you haven't got the sometimes awkward job of refilling and removing the air and having to run all over the place venting rads.