I take it you've never tried the likes of an Aqualisa shower fed 22mm
from a gravity fed system with a reasonable head?

The problem is that the gravity system can't provide the pressure. OK, if
well designed, they can provide good flow rates (i.e. for a Victorian style
drencher), put you'll never get a nice pounding from the pulse function
without a bit of push. That needs at least 1 bar, preferably more. That's
over 10m of head. Many people can't afford to buy an entire block of flats
so that a tank can go on the roof.

Christian.