Wow, big post. I'll just hit a couple points to respond to. ;-)

Automotive engines today are exceptionally reliable. No where did I
impugn automotive engines. It is also obvious that all readers are not
clear on the use of the adjective "many". Many does not mean all or even
imply a majority, it at most implies more than "several".

I may have jumped in without fully understanding the response to the OP.
That never happens around here, does it? g

Interestingly, it is most always not the core auto engine that
experiences failure when an auto engine conversion has problems, but the
prop speed reducer, or fuel system, or non original fuel system, or
whatever else has been added or re-engineered by the experimenter.

What the heck does that have to do with it?

I was making a point that the experimenter that builds his ow PSRU does so
at much risk, unless it is really well engineered. There are comercial
units out there that have very good reliability records. People get into
trouble when they try to improve on the standard conversion, usually. You
are correct though, the engine and PSRU and any other additions that make
the whole package is what really counts in the end.

I am not a certified engine bigot and I believe experimental aviation
should be experimental ( I also believe homebuilts should be homebuilt as
well) I was more specifically aiming at the vendors targeting the
experimental market with uncertified engines that make outlandish claims
regarding reliability, fuel burn and most especially power.

I get a kick out of the 100 HP VW's, especially.
--
Jim in NC