What disadvantages are there in having an entire network running in 100BaseX full-duplex mode?
One disadvantage can be cost. Every full-duplex device needs to attach to its own dedicated switch port. You cannot have multiple full-duplex devices attached to the same port. If you have many devices, you need many ports, which equates to increased cost.
Another disadvantage can be congestion at the servers. As you increase the number of devices operating in full-duplex mode, higher amounts of bandwidth hit your servers. This amount can greatly exceed the bandwidth capacity of the server connection.