When I try to send a message to a device it works for laptop/PC devices using the website but it does not work for a Smart TV (Roku).
I've literally stood in front of the TV and from my phone sent a message to the Smart TV client and it did not come up on the screen while a movie was playing. This send message to device feature does work on Android Emby App (smart phone) and laptop/PC using the website. I am not certain if it works for Apple Emby App however as I do not have any Apple devices.