Having been on both Voith Schneider and conventional tugs there are several main differences.
The Voith units are less efficient at steaming as there is considerably more drag and the sculling motion is not as efficient through the water.
The depth of the units also give the tug a harsh pendulum effect which makes them very uncomfortable. They recover very quickly.
Most Tractor tugs will be used astern as the Voith units are positioned 1/3 of the length from the bow, this produces a wash that will reduce the tugs efficiency against a ships hull.
The Voith are far more maneuverable but at a cost. The units are more expensive to buy and maintain over their lifetime.
This is why Azimuth powered tugs have become by far the most popular - cost.
Small harbour tugs suffer from excessive draught so cannot operate in shallow waters.
I have been aboard when the Voith unit picked up a cable from the seabed - not nice!!!
The Voith Schneider units were developed for maneuverability and are very good at that, but there is a heavy cost associated with them compared to Azimuth so owners taking the cheaper route.
Bob