But again, if the appliance is fully-fitted / built-in, there simply
*isn't* a back for anything to fall down!

A fully fitted/built in washing machine?

Yes. I've got one of those. And a tumble dryer.

Or dishwasher? Even that vibrates.

Not much at all. Oddly, I've got a fully integrated dishwasher too. Even the
washing machine, which is 1600 spin, hardly vibrates at all. I'm hoping that
the extra expense on the integrated version went into vibration damping, but
suspect it just went to the shareholders. :-(

Or a fridge or freezer - which NEED space for heat to escape?

Yes. They are correctly designed with airflow in mind. They give various
options in terms of vent sizing and positioning. They've thought it all out.

Integrated appliances are all designed differently. My fridge and freezer
are bolted into a specially designed carcass, much like an integrated oven
would be. The laundry and dishwashing appliances are basically freestanding
units that can have full size kitchen doors attached. They can be pulled out
from under the worktop like a freestanding appliance and aren't bolted in
place at all.

Christian.