I figure it's a little bit of both, remember most of the portrayal that we've gotten of the Reavers, is what the Serenity
crew knows of them. Like someone else mentioned, one hardly needs to be 'smart' and 'civilized' to function and operate technology. Just because they were nuts, doesn't mean they couldn't use technology or function as a proper group. That's likely what's to make them an actual threat to anyone, otherwise how could they be?