Not to get off track but:
I have never used anything personally on a heli but a OS-
I have some with 700+ flights on them, which tells me there is a lot of QC (quality Control) put into every engine out off the assembly line.
But on my Fazer Fun Fly I had a very strong running MVVS, it would hover into the wind as advertised by a hobby store selling them. I as able to crank it by just a flip. But after a while it began loosing compression prematurely..
I use heli engines now on my FF's, a OS 46 with the newer carb body has not once let me down-Now pilot error in a hover, well there have been some issues there.