My understanding is high compression engines run very hot and need to be built beefy.
They also create a lot of NOX.
Still waiting for Mazda to add the water injection of a
https://en.wikipedia.org/wiki/Six-stroke_engine, the long-stroke reduction of
https://en.wikipedia.org/wiki/Atkinson_cycle, and a variable combustion chamber of a
https://en.wikipedia.org/wiki/Variable_compression_ratio.