There may be disadvantages to turning all units on or off simultaneously.
Examples:
* If an accident happens it could damage all units.
* If a fuse blows then it is not obvious which unit caused it
* possible increased intensity of spikes and surges
* rise or drop in voltage may affect sensitive equipment elsewhere
Off-grid systems sometimes use excess power to heat water. Is that the purpose of your resistive elements? What if shielding corrodes and water comes in contact with an element? Do kiloWatts of power go through pipes to distant places?
Equipment may contain capacitors which charge rapidly on power-up, often drawing surge current. When several units turn on at once, it means greater surge current.
Or equipment may contain inductors or transformers which generate a high-voltage spike when current is abruptly shut off. Properly-built equipment should contain safeguards, yet these can be defeated by age and wear.