1) Can't be automated without a failover switch to disconnect the house from the grid so it doesn't back feed into the actual power lines outside the house and either break or risk injuring linemen who think a line has been disconnected and isolated.

In addition to that you're feeding energy back in through a much smaller wire so you're limited to the capability of the circuit it's connected to and the devices have to be smart enough to actually limit their output to what the wall can handle. Otherwise imagine you have 2 of these each on 1000W circuits (not a normal circuit but for convenience lets pretend) and you have 1500W of load running elsewhere in the house. One runs out of power because it had a lower charge state when power failed, now unless the remaining UPS limits it's output and browns out everything you're pulling 150% the rated power of your circuit, potentially damaging the wires or the loads you're powering.

This is potentially what's causing fires on nVidia's high power connectors, one connection wears out or is simply loose so more power flows through the remaining pins and it's too much for those wires causing fires/melting the wires.