Testing at those settings assumes that manufacturers know that they impose the greatest stresses on the engine. If they knew that, it’s hard to explain why some POHs (albeit old ones) would still recommend operations at those settings. Sure, it’s the setting that will make the aircraft cruise very fast, but it’s not fun for the engine.
Possibly because they are designed and engineered and tested to be able to run at those settings for the TBO of the engine?
There are also some assumptions about what is hard on the engine - specifically heat and high cylinder pressures. But if they were planned for in the design, are they really hard on the engine? If you are within limitations, not necessarily. Operating in ways that were not anticipated when the engine was designed may be harder.
Other things that could be hard on the engine:
- Long periods at low rpm. You may have less than optimum oil circulation etc.
- Low temperatures. Lead scavenging requires high temperatures to work properly. Lead deposit problems have been described in low compression engines operating on higher lead fuels than they were designed for, and Rotax etc. with liquid cooled heads.
- Low manifold pressures reportedly may cause problems with ring seating.
My personal opinion is that you are least likely to have problems if you operate in the way the designer expected.