Unless the frame is rusted or some weird junky off brand, it's not usually the frame that's the problem, the rear shocks take a beating, the cup and cone bearings on a lot of mopeds are a weak link, as are the spokes and rim. Some mopeds fitted heaver 10 or 11 ga spokes, but some of them are 12ga.
The express have a really weak rear shock and the rear wheel is also kinda marginal. Not sure what to make of the single side swingarm.
It's got a lot to do with how you ride as well, I've ridden with some people that don't pay attention and just hit everything, potholes, etc. If you are watching the road and can avoid the major bumps or ride in a relatively smooth area you should be fine.