In all the O365 adoption projects where I have the pleasure to be involved, there are always questions about how the network will affect to users. It is necessary to consider that for business is a big step to go from resources allocated On Premises to all their resources allocated somewhere in the cloud.
So, it is normal that the IT department will be afraid when they must affront this change, more in particular, as I said at the beginning in the network part. In general, doing a network assessment, will clarify the vision to the clients and, they will be more confident about the change.
But, it is recommended to give them a series of guidelines, to understand better how works the suite and which networking elements will be involved in it. I strongly recommend the following post in technetcommunity about this topic.