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.
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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.
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That is not at all the whole story though...
When the Merlin initially came out, it had an expected combat life of just 20 hours.
Which was continually reworked and revised thoughout the war,
the horsepower likewise being considerably developed. Considerably.
The Napier family of engines also had considerable development - and initially were quite unreliable.
Didn't the story go that Napiers got acess to Bristols' development in sleeve material and technology,
with different management, which eventually improved it out of sight.
The early versions were tried with aircooling, but w/c was required to be reliable.
It was the later H16 Napier Sabre engine that was said to be the most powerful piston engine about,
although at 5500 hp it was only just ahead of the later versions of the Merlin.
Perhaps worth repeating here too a story of the Napier sleeve valve engine in the Hawker Typhoon.
There were apparently known for sometimes being somewhat difficult to start, and catching fire in the process.
So much so that one early squadron of Typhoons had handwritten written near the engine
"If this catches fire during starting, don't just run around waving your arms,
do something about putting the bloody thing out."
A fearsome weapon when they were on song though apparently.
Capable even of catching doodlebugs.
Off track slightly though.
And the B&S was somewhat before this, with a much more modest output and capacity..
