The early Tower engines had a big problem with an intake leak on the carb, it took time but they fixed that. The early OS FX line had a bad problem with the chrome peel of the cylinder, it took less time but OS fixed that problem. The SK engines I use are ringed with a tapered cylinder but as John mentioned, the engines from China can and do have a quality control problem. I had the crank pin break in one SK .91, the engine was new and it only took a week to get it back from the warranty service. I was told by the service rep all the engines from China do suffer a quality problem.
The new AX line of engines from OS can't be beat except for the price. I even like the TT PRO series of engines and when I was using the smaller engines I had great luck with them too.
Someone new to the Hobby should go with OS if they can afford it, you can't beat the quality and service if something does go wrong. Service and quality are worth the money. Those of us that have been in the hobby a while can get away with some problems but for new to the hobby people they should have trouble free equipment, that is OS.
If money is a big problem then there are some good choices offered here.