That's like saying an internal combustion engine should be simple...gas goes in and torque comes out

Gas combustion creates poisonous carbon monoxide which should be vented to the exterior, so you'd also need HVAC.

Did you check the code on the clearance for combustible mantels?

Both are formed by burning fossil fuels; CO2 is ALWAYS formed, but CO is only formed during incomplete combustion.  

I paid $1500 each for two units last fall, plus another $200 on one for some code related upgrades for combustion air.

Where we operate that means using a non combustable barrier (such as dense mineral wool) in the floor cavities with 1 (or 2) layers of 5/8 fire rated (Fire-X) drywall on the underside ceiling.

The combination of Al wire and an FPE panel is especially concerning because Al wire is more likely to have a loose connection that will overheat to a temp that can cause combustion, and the FPE panel has a high chance of not tripping when that happens in order to prevent that overheating from continuing.

I've swapped out 3 such furnaces and replaced them with sealed combustion furnaces.

Tell that to all the millions of people who lost their life savings and retirement in the dot-com bust.

Keep in mind, medical marijuana comes in different forms and is not always consumed by combustion (smoking).