I am using the back of my 4 tube 32 watt lay in fixtures for the seed shelf.
I have 5 shelves a tatal of app 600 watts of light/heat/energy.
In my fully insulated shed it is 70 degeees when the outside temp is 50 degrees. I have a oil filled heater which comes to assist if the flourescents cannot keep up.
Anyway, why waste the back of the fixture yes heat rises.

I personally prefer the heat mats for a couple of reasons. I can thermostatically control the heat mats. My heat-mat thermostats have sensor bulbs that I can place in seedling pots to avoid overheating.

Also, I can control the heating independently from my lighting. Most plants grow spindly if they are cold at night and warm in the day, because the "DIF" figure that represents the daytime temperature minus the nighttime temperature is fairly high. A DIF of zero or even a negative DIF produces more compact plants. I can put my heat mats on a separate timer to run them at night and even turn them off during the day.

I have some plants growing in a cool breezeway. There I can just plug the heat mats directly into the wall socket current to keep them on all the time, which keeps the plants from getting too cool at night. I couldn't do that if I were depending on the fluorescent fixtures for heat.

Of course, heat mats and heat mat thermostats are somewhat costly, and that can be a significant factor arguing against them.

MM