Emerson Exchange 365

The first reason is that almost all projects are executed from the bottom up, meaning you do base layer stuff first (IO Modules, Graphics, etc) because that information is already know (I/O database, P&IDs, etc) while the control and batch/sequencing is being designed. Second would probably be what gamella indicated is that it does require a license. Third might be that people don't know this functionality actually exists as I find this many times with other functionality in DeltaV.

I got some hints from other sources.
One issue is that unlike EM devices, you cannot Ignore Unit devices. This increases complexity if you have small variations between units: either bind a dummy device, or you need to create a new unit class.
The main disadvantage is that it increases the interdependence of objects, both in terms of VCAT and of downloads. I cannot describe any particular examples, but basically you'll need to check out more objects than normal if you are trying to change something and you might be unable to download just one instance of an EM, the system will force you to download all. I'd recommend anyone planning to implement Devices in Unit Class to have a look at these aspects before going too deep in this direction.