I have the same problem with the modern (Star Trek III and later) Klingons that you have with the Reavers: I don't believe for a moment that these people are capable of inventing the light bulb, much less achieving any ongoing technological culture over periods of centuries, because they're just too stupid and violent.
So, I guess there's always some suspension of disbelief involved.
I'm not sure this is exactly a plot
hole, in either case; it is a conceptual weakness in both, though.
BTW. the "rules of the franchise" are not that ships are "incredibly difficult to fly and maintain."
The rules are just the opposite - craft like Serenity
were mass-produced for purchase and use by settlers striking out from the civilized worlds to make lives as farmers and herdsmen on less developed planets. We see that a young war vet with a little money can buy one secondhand in order to launch a small cargo (smuggling) operation, find a young woman with an intense interest and native skills but no formal training to maintain it as the chief mechanic, etc. These things are nearly the equivalent of trucks or small steam boats. So while you have a point - it's hard to see the Reavers keeping a big rig on the highway past the point of needing to change a tire - the ships are anything but incredibly advanced or challenging technology in the Firefly
'verse. They're workaday transportation.