For example, the maintenance costs for a 80 ton bollard pull Rotortug are on average 10k €/year higher compared to Z-drive ASD- or tractor type tugs. This is due to the special survey costs for the third thruster after 5 years. Filters and spares for smaller engines are generally cheaper.
In return you get a level of redundancy where you can safely operate your tugboat in case of engine or thruster failure and maintenance, especially when operating remote terminals. It is better to have one tug operating on 66% capacity than none at all.