I do not think you will have any issues having your call applications on Unity, we have approximately 20 different buildings across the country using the same Unity for voice mail as well as Auto Attendants etc and it works very well (close to 5000 people and growing every day as we add people and convert locations). Some of the major advantages are that there is only one system that needs to be backed up, maintenance is minimised, as are hardware costs, and, once you are used to it, it is quite easy to do.
We went from a primarily Nortel house, to Cisco in just a few years and even though I did not think it would be a good idea initially, it worked out very well.