....and I've used it maybe a half dozen times in 30 years.
Sometimes, like after an engine rebuild....replacing a
busted crankcase....it's nice to put the motor in the stand
and run it for a few minutes....just to see if the darn thing
will run. That's about all the "testing" I need on the test
stand.

I can see however, the usefullness of running a high perf.
motor....like a Jett, or a Nelson....ones with a super tight
"pinch" for a few tanks....just to let the piston and liner
make friends....without the risk of leaning-out in flight, and
overheating the motor.

Personally....I don't see the usefullness of running a sport
motor on the stand for break-in. To me it seems like a waste
of time and fuel....when I could have been flying the plane,
and letting it break-in on it's own. And "testing" it on the
stand will not tell you how it's gonna run in the plane....
especially if there is a problem....like with an inverted
engine. That time could be better spent running the engine
in the plane....and getting it set to run where it needs to
be.

Usually on the stand....the tank is higher, and the fuel lines
are longer....ect.

With a ringed motor....like an OS, or a K&B....I'll run it in the
plane at a rich 4-cycle for 5 to 7 minutes....peaking it up a
few times, let it cool of a bit....fuel it up....and go.

On a non-ringed motor....same thing....but I'll run it 3 times
and go....only because the needle settings can change quite
a bit on the early runs....and I want to make sure that it's
not gonna change and overheat on me.