I am using MySensors along with Vera to control all the lights in my house. This has lead to many sensors on a single Mega 2560 node. For example: 16 relays, 5 PWM LEDs, and a scene controller with 7 scenes.
I've had a lot of problems where messages clobber each other, particularly when e.g. a scene is triggered that changes several of the relays or PWM LEDs. From what I can tell, when the gateway sends a command to change a light value, it requests an ACK in response. This causes the sensor node to reply back to each command. When many commands are sent in a row, these ACK messages end up (I think) colliding with the messages that the gateway is trying to send. This results in dropped commands that are never received by the sensor.
An easy example is to create a scene in Vera that turns on all 16 relays. The first few commands make it to the sensor and their acknowledgements are sent back. The next few commands never make it to the sensor. Then a few more make it through.
I had assumed that the MySensors library had some form of message queuing, but perhaps that is not the case? I never have any issues when just one command is sent (radio is very stable, etc.).
In case you are curious, my code is here: https://github.com/TheOriginalAndrobot/Arduino_Projects/blob/master/Living_Room_Controller/Living_Room_Controller.ino