From what I've seen of them, they can be really finicky to run and don't have a lot of power which they actually do need to overcome their less than efficient geometry. Really easy to build them with a bit of bind in the system, so tolerances are best kept a bit on the loose side, which again makes them often be a bit weak when running.
A really good machinist, who sets them up square and with proper tolerancing, can make a good one that runs very well. Seems that may be the case with this one here!
NICE !!!