I may be wrong (frequently am!) but if you are running multiple MDs simultaneously, then contention will become an issue.
Most modern Cat5 networks are now fully switched. This means that each device has dedicated bandwidth to the switch device and this bandwidth is duplex (so can transmit and receive at the same time).
Powerline systems work more like the old 10Base2 system with a contention based, shared media. If two devices both try to communicate at the same time, there will be a collision and both will back off for a random period of time before re-trying (the theory being that since the back-off is random, they won't collide again). As the number of devices increases, however, so does the chance of a collision. The result is that you get less than the total bandwidth available as well as dividing the available bandwidth between the devices (and not even a fair division!)
Overall, it may well work, depending on the number of devices and the simultaneous usage, but you won't know completely until you try. Even then, you may have odd problems occurring from time to time, which will be difficult to reproduce and therefore trace.
If you can, I would use Cat5 anywhere you can and only use the powerline in places where it really is impossible to run more cables in.
The lesson to any future builders is, ALWAYS leave conduit in the walls to allow cabling to be run at a later date without needing walls and floors to be ripped open!
Just my 10 Eurocents