Components on floor

You should be ok as long as you use some type of isolation under each unit. Springs much preferred or at the very least soborthane. No spikes! Anything place directly on the floor, regardless of floor material, will be an invitation for unwanted vibrations.

It shouldn't cause any negative problems.  The installation method of your floor will determine exactly how prone it is to external vibration, but its mass is quite high compared to shelves or racks.  It would take high amounts of energy to get the floor moving.

nothing to worry about outside of tripping over wires if not managed well

Wall mounted electric fireplace? What is that... does it have motors and re-stats? It might create some interference...

I agree that isolation would be necessary. Springs at least. The floor is seismically active and will transmit the truck and car traffic, actual seismic activity. I have a seismometer in my house. It is surprising the difference in background vibration between day and night. This is why Townshend Seismic platforms (under speakers) are so effective... but also important for components. 

 Is this a bad idea, and if so, why?

Yes.  You have to bend over to use them.  Must you put them there?