bluskis,
Not quite sure what your definition of mass produced is; a lot of well-known GA machines are built in disappointingly small numbers these days.
Several microlight types available in the UK are factory built. Probably the majority are kits. All of them are subject to certification and annual inspection under the auspices of the BMAA, and ultimately the CAA. It's very similar to the way in which the PFA deals with airworthiness for their aircraft.
Any aircraft can of course suffer from "nasties hidden in its structure" but that's what initial certification, pilot checks and annual inspections are there to spot. I'm happy to say that it has been many years since the early days when some (then unregulated) microlights did suffer in-flight structural failures.
Range and payload is as ever a matter of horses for courses. One-up, my old CT could legally carry enough fuel to take me about 800nm which was more than enough for the human bladder to endure. If I want to take a couple of friends plus a bit of luggage then obviously a microlight is not the right tool for the job and I hire something bigger, but many people do carry out some fairly epic touring flights even in flexwings.