When I first went ‘up the road’ in 1967 the company I worked for had the choice of Gardner in an ERF or Gardner in an Atki and later Gardner in a Guy BigJ the engine in all cases was a 180hp and they all used oil at the rate of a pint per 100miles + a pint (probably lost through the startup smoke).
They did seem reliable but noisy and slow, about 57 mph max on all the above if I remember rightly, and after a while you learnt what gear and when to change for every road and hill on regular journeys. To be fair speedwise there was a 40mph speed limit everywhere, dual carrageways included, except for the (then) limited motorway network so maximum torque at 56 mph wasn’t a requirement then.
I think the ultimate demise was mainly due to Gardners assumption that turbochargeing was a soon to be forgotten gimmick.