They probably are very good at it. It helps that they have some interesting spells that might help with it, like
Yeah, but only if it was illegal. The fact they have the concept of "muggle hunting" doesn't exactly speak highly of their attempts to police such behavior.
The only mention of "Muggle hunting" I am aware of was in the context of one of Sirius's relatives, who at an unknown point in the past tried, and presumably failed, to make it legal. Sure, people like Umbridge probably wouldn't give a shit about Muggle Rights, but due to the Statute of Secrecy if nothing else, I expect this sort of stuff would be illegal.
Of course it still is useless for things like love potions. Or more innocuous things- they have a spell that makes the target happy. Use it to condition someone and you can get them to fall for you.
Again, I don't doubt that, realistically, crap like this would happen sometimes. But I don't see any evidence that its as common as some here seem to think, and I certainly don't think it would be officially tolerated.
Really, it is a miracle the statute of secrecy survives in the series and you don't have rogue wizards. Never mind being a dark lord- just start your own cult.
Actually, there are some known incidents of SoS breaches that were not completely covered up. I'm pretty sure Rowling's companion book Fantastic Beasts and Where to Find Them
claims that the Loch Ness Monster and the Yeti are cases of magical phenomena that are not well covered-up, and that the countries where these incidents occure have gotten in trouble with the rest of the magical world as a result. Unfortunately I don't have the book around right now, so I can't provide an exact quote.
But as for rogue wizards starting a cult, something that public would probably get noticed and said wizard would wake up in Azkaban one day.