I'm not convinced by any means that sidevalves "need" higher temperatures; I don't believe they can get rid of it the extra heat by virtue of the shape and mass of their main castings.
Lest anyone else has forgotten the basics of motor design, let me remind everyone that a motor's efficiency is a matter of how much power it derives from its fuel.
Compression and heat are synonymous when breaking down fuels and extracting as much work force from their combustion as possible. "Cracking" the chemical bonds of the large molecules provides the energies that move the pistons. The pistons are rebounding "hammers", and the heads are the "anvils".
Flatties use residual heat more than compression to achieve this. Thus they can use lower octane fuels.
OHVs rely on compression, so that they can reduce residual heat for the benefit of hardware, however much cooler-burning fuels must be used.
If either is not up to its optimum temperature, its efficiency is compromised.
If these temperatures spike, the hardware is compromised.
Needless to say, many have tried to maximize the compression of Flatties beyond their capacities to cool themselves.
This is like training Clydesdales to run like an Arabian. It isn't in their genes.
But this is by no means a valid reason to condemn them, as 45Brit is so often eager to do;
Instead, their strengths should exploited, especially in these times of poor fuels.