If you use Ethernet or some other kind of a network for communication, any controller with an Ethernet module can act as a server or client in such a network. Downside is the requirement of a controller on every node, that translates the commands into outputs to the actuators. It's up to you to specify the number of actuators, connected to each network node. Here a practical limit is the number of wires, and their length, required to drive the actuators. It may be handier to use multiple small controllers in a network, instead of one big Mega. No network cable and connectors are required to attach sensors and actuators to a controller, you can use whatever cable fits your needs. Control cables with more than 50 wires are available, but they are not very handy and useful when finally relays in different places shall be connected.
Next comes the power source for each node, for operating the controller, Ethernet shield, relays etc. PoE is possible, but only with one voltage and limited current. AFAIK it also requires a splitter, to separate the power from the signal lines. In this case every node should have a fuse on its power line, so that an inadvertent short does not shut down the entire network supply. If higher voltage or current is required by an actuator, a local power supply is required on such nodes, at least a step-up or step-down regulator, or a wall wart or other mains supply.