To lump all sleeve valve engines together and apply conclusions drawn to all is just as wrong as doing the same with poppet valved engines.
R's comments relate to the double sleeve White/Minerva/Daimler/Knight car engine. This had three crankshafts, one for the conrod, and one each for the inlet and exhaust sleeves. The sleeves were concentric, reciprocated straight up and down (thus concentrating wear) and had tiny letter-box slots for ports. Very quiet, of humble performance, and due to the wear actions and multiple boundaries, given to smokiness when worn. Design lends itself to in-line engines.
They're not to be confused with the single sleeve valve of Barr & Stroud/Argyll/Bristol/Napier and Rolls Royce. The single sleeve oscillates in an elliptical path, has large ports that open and close rapidly and are not obstructed by valve heads. As 33d6 rightly points out, the sleeve action is near to perfect for spreading both wear and lubricant. Breathing is excellent. Volumetric efficiency of a B&S exceeds that of a current model four-valve per cylinder Nissan.
There are complexities in making a single sleeve valve in-line engine. Argyll used skew gears and a shaft initially, before going over to a wobble shaft in later engines, but both options were expensive to make. Singles, twins and radials were much easier.
Single sleeve valve engines hit their peak in WW2 aero engines. Bristol Centaurus, Napier Lion, and I forget what the "H" pattern RR was called. All rendered obsolete by jet turbines, but still the most powerful spark ignition engines ever built.
Where the RAF overhaul period for a poppet valved aero engine was 500 hours, for a ssv engine it was 1500 hours.
The limiting factor on B&S engines is the carburettors of the day. Rapid throttle opening results in hesitant, lean running until equilibrium is returned, far, far worse than the same effect in a poppet valve engine, because they breath so deeply. Carbs with an accelerator pump, even better, fuel injection, would solve this.
Fewer than 2,000 motorcycle engines were made, and the vast majority were 350cc like Wetdogs. They were used by almost all of those manufacturers reliant on proprietary engines, from Brough Superior through Rudge. By their nature they were more expensive than the rival offerings from the proprietary engine floggers, but apart from their being regarded as "unconventional" at a time before "convention" had been established, there wasn't much wrong with them. However, unlike all of the other proprietary engine builders, engines (or even motor trade) were not B&S main line of business. They were, still are, precision optical engineers and manufacturers; binoculars, periscopes, rangefinders etc. This business of course dried up with the armistice of 1918, then vanished completely as ex-WD stuff flooded the tiny market that was left, and so they sought a slice of the burgeoning engine market. By 1927, the WD work was coming back, and engine manufacture ceased.
