Yes it would appear that the Magnums are a lucky dip. All but two of mine have been purchased and used in Australia and I've had no problems at all. Maybe the scene here in the US, Where there is a lot more fliers than in Aus (Eg - within a 20 mile radius of where I live there are at least 5 clubs all with significantly larger memberships than any of the clubs near my home in Perth) so I guess that the overall number of "dud" products will be greater than we hear about in Australia.

I've never owned an OS 4 stroke so I can't comment on those (I was lucky - I nearly bought one of the problem 70 size ones a couple of years ago when they were "on special" but they were all sold out before I got to do so). Everyone I know with them is happy with them and they don't seem to spend much time "tuning" them at the field.

Compared to the Saitos, the Magnums probably are a little more fiddly to set initially and they do require a bit more setting whilst they are running in but once run in they are pretty good - I am very fussy about using fresh & filtered fuel on all of my planes and am also fussy about accurate measurement of all of the fuel components when I mix it 20 lt at a time (I weigh the ingredients into the mixing container) (Between the three of us we go through around 5lt each weekend).
I also insist on using a Tacho when running in and setting up my motors - Then once set I am more interested in flying instead of playing with the motors (unlike some people that I know who spend most of their day "Adjusting" their motors to compensate for their poor throttle thumbs).

For 100% consistency in good quality and excellent power its Saito all the way for me when it comes to 4 strokes. For a generally good quality product at an excellent price - It's Magnum. (I can't recall anyone actually having been stuck with a bad Saito and can only recall hearing of a couple of incidents where people have not been happy but they've had their engines replaces quick smart under warranty - I am sure that there are exceptions to this - I just don't recall hearing about them - maybe my Olztimers disease is setting in - like me confusing you with another poster that bought his Saito 125 at the same time I got mine)

BTW - I don't buy Aussie cars either for the same reason. It's BMW, Landcruiser (for the horse float) and Kia (Van for the models) for us.