There's another category of sorts to be added to the above - strictly user dependent ofc...
While with the cheap 2 strokes it's completely possible to just get a "lemon" - a dud that was just assembled poorly, with a lil preemptive work & upgrades I've found that they can also be quite reliable, if purchased from a reputable dealer and care used while building I think you're pretty well covered - I've over 5000 miles of happy abuse on my first build so far and it's still going strong.
I suspect that if you go through the "cheap" kits like candy, a more expensive engine may not be much more "reliable" as you're missing something - like the basic, normal maintenance ALL small, overworked 2 strokes cobbled to a bicycle would need. Not that there's a lot even with the cheap'os, but so much depends on the initial setup - and that
both the cheap ones and a higher quality engine would have in common.
Don't get me wrong - naturally a better engine would be "better" lol, but I seriously doubt engine manufacture alone is the primary
cause of the reported failures. While I too would be willing to buy an engine of better manufacture - it's also true that ya can't simply buy your way out of maintenance