if you're looking for reliability, get a moped. if you buy a used one that runs and hasn't been messed with, it'll usually keep running forever if you take care of it.
a 70's quality moped like a puch or tomos is virtually indestructable. the motors are built by experienced people that cared about their products, and the rest of the bike was designed and built with longevity in mind.
a motorized bike is fun and you can build virtually anything you want, but most of us aren't professional vehicle designers with engineering skills. engine quality is hit or miss, you constantly need to repair, replace, tune, tinker, and find the tools you threw across the garage in frustration.
but it's still fun...