I have 2 "no namers" currently. My first build was a panel tech 48cc from Amazon, $110 I think it cost me. Had some problems initially, both magneto magnet and magneto coil came factory installed wrong, and with a washer stuck between the two. It was my first build and I didn't have the knowledge I have now. It took me days to figure out the problem, I kept thinking it was something that I did wrong, when really it was the fault of the factory. Other than that the engine was pretty sweet. After break in, I beat the piss out of it and it kept on a running. A year and a half or so after building, the crank case cracked, (probably my fault, loose chain...). Planning on rebuilding it this winter when it's to cold to be out riding.
I just completed my second bike. Another "no name" El cheapo engine kit, this one is a 66cc though. Also from Amazon. l put it on an old Murray beach cruiser ladies model I just chopped. So far so good, but to early to tell. This kit was a bit more expensive than the last, and it shows. Has Allen heads for all engine hardware instead of those darn flat heads, and came with a much better chain tensioner than my previous kit.
Long story short, from my experience, you get what you pay for. If you got the money for the more expensive kit, I would go for it. But at the same time, the cheap "no name" kits have been good for me so far. That's one of my favorite things about these cheap China girls. If they ever break beyond my repair skills....I'll throw it out and build another, and not even break the bank. I wouldn't go as far as to call them disposable, but they kinda are. Ride em till they burn.