He is liar and a sophist.
His ideas are also idiotic, but both should be treated separately.
He is a lying sophist because he doesn't practice dialectic correctly¹ and because he straight out lies².
His ideas are idiotic because they do not take roots in reality at all. One sentence can resume everything you need to destroy his ideas "correlation does not imply causation".
However if that's not enough let's develop; stefan assert that the government intervention in his classical fields of intervention, (education, economy, employment, health, wars, immigration) are an "intrusions into personal liberty"³. Obviously when you are honest and listen/read Stefan Molyneux for 5minutes you realize that the core of the problem seems for him to be government and (monetary) imposition. Stefan Molyneux however accepts that two individuals can contract obligations toward one another through open (non-coercive) trade (what he refers to as : the free market)
So here are the reasons why his ideas are idiotic:
1 - What Molyneux doesn't understand that the government exist because of the population willingness to accept its control ⁴. Stefan completely forget the intrinsic role of the government as regulator to picture it as a coercive monster that only works for power but actually there is a social contract between people and governments. So Stefan is for contractual obligations in one case and against it in the second case.
2- What Molyneux doesn't understand (I believe by a deliberate choice) is that in an unregulated market (such as his free market) people create something we call a monopoly and under monopolies there is inherent coercion, hence the historical creation of instances of economic control (government - what he'd know if he read a history book once in a while). Therefore government does serves has a predatory force against the populations but has a protection against predation.
so if his doctrine takes root in ideas that are demonstrably wrong, and gives unequal results it is fair to assume it is wrong. If it is wrong and it took two seconds to make the necessary research it is fair to say it is idiotic.
¹
https://www.youtube.com/watch?v=dO1P5wlW-HQ²
https://www.youtube.com/watch?v=4-67C2htgI4³
http://archive.lewrockwell.com/orig6/molyneux1.html⁴Alain - "le culte de la raison comme fondement de la république" conf. 1901