I used to sell used engines and I have had a lot of different types and brands run through my hands. I would say there are good and bad engines from every manufacturer and the best way to keep from being burned is customer support. In my experience Magnum customer support is excellent and Sig Aviastar also. I had a minor issue with a new Magnum XL15 and the engine I got back was simply fantastic!
Having been a Quality Control Supervisor I can understand why things go wrong so easily, the true test is how well those issues are resolved.
You should not base your opinion on one engine from any manufacturer since that one may be a poor example. Recently I have had 5 different Magnum engines to test and one had only a slight problem that was quickly resolved.