What's wrong with this engine?

I was perusing some lists of Ultralight engines and noted that
the Limbach 275 was listed as 20 - 25 hp and 17 lb. I figured
that was a typo, since it was less than half of the weight of
other engines of comporable power--more likely it was 17 hp at
20 - 25 lb, still good stats. But I've seen it listed in three
more places as only 17 lbs or thereabouts including he

http://www.ultralightnews.com/limbach/limbachl275e.htm

Holy crap! How does Limbach do that?

Now, since it develops max power at 7300 rpm you'd need
a 2:1 speed reduction or so to use a 48" prop, effectively,
right? Supposing the speed reducer DOUBLES the weight, it's
still pretty darned impressive. According to what I can find
on the history of Limbach, the design is based on a VW, but
is built from the ground up to be an airplane engine.

So, what is the down side to these engines? Do they cost a
gazillion dollars or blow up at 20 hours or something? There
has to be a downside somewhere.

--

FF