Why? Be specific and technical.

Where is your long-term field study comparing LOP and ROP operations?
How many engines have been monitored in service from start to overhaul,
under what conditions, and for how long? Have you shown a
statistically significant difference in MTBF, service life, or cost of
maintenance? That's really the only way to cover all bases. Sometimes
this is not practical, but lacking a long term field study, you at
least need a reasonable model. A compelling model would address the
following issues, as a minimum:

What are your parameters to asess engine roughness in normal LOP and
ROP operations? How do you model the imperfections caused by pilot
technique? Do you have amplitude and frequency data on engine
vibration at various mixture settings? What kind of sensors did you
use?

Do you have long term operational data or at least a model showing the
long term behaviour of the engine mounts, bearings, cases, crankshaft,
etc. under the vibration conditions? Without long-term operational
data, I would expect at least an FEA.

Do you have any information at all on the differences in combustion
end-products in excess-air vs. excess-fuel combustion reactions? I can
assure you they are differrent. Are any of the combustion products
harmful to the engine components long-term? Do any pose corrosion
issues when the aircraft is not flown for several days or weeks, as
commonly happens with private planes?

How about that big mixture pull - it takes the mixture through peak.
What is the effect of this transition on the crankshaft? Some analysis
of this issue was done in the 1940's, using the limited available tools
- but only for radial engines, which have significantly different
crankshaft designs. For that matter, about the only large base of
operational data in the LOP regime comes from radial engines - which
are different - using 1940's and 1950's fuels which were significantly
different than what you're burning now.

I'm sure given time I could think of other issues.

The main arguments for LOP operation are short-term economic ones -
less plug fouling, lower fuel burn. These are pretty compelling. As
for effect on TBO and general engine longevity, there has been much
hype and no compelling evidence.

BTW - to answer your other question - I run the R&D group for a major
manufacturer of industrial instrumentation. So yes, this is pretty
much right up my alley.

Michael