[The words "crazy", "insane", "delusional", "corrupt", and "crooked" redirect here.]
Libertarians can be divided into two basic groups: "cosmotarians" (example: Reason Magazine), and their country cousins like Lew Rockwell. The former group leans to the crazy and corrupt side, while the latter group's craziness is more principled.
Another split is between "big-L Libertarians" (those in the Libertarian Party) and "small-l libertarians" (the rest). The difference is that the former group's insane delusions are codified, while the latter group's insane delusions are wider in spectrum.
There are also those like the Tea Parties who have subsumed libertarian ideas. True libertarians tend to dislike such people, especially if they combine libertarian-leaning economic ideas with social conservatism.
One major reason I oppose libertarians of all stripes is due to their unworkable position on social welfare. Most Americans support social welfare programs broadly defined, and even more would support them if they were taken away from them. Imposing libertarian economic ideas on the U.S. would not result in their addled utopian visions. Instead, millions of Americans would face undue hardships, perhaps resulting in riots and even some form of socialist revolution. Taking a little bit of socialism away (the current system) could end up in a whole lot of socialism.
Libertarians also tend to favor free trade, something that's had benefits for many, huge benefits for a few, but that's also had a negative impact on millions of Americans. The cosmotarian types support that out of ideology but also because their benefactors stand to benefit from it.
Another reason I oppose libertarians is because they tend to lie and smear. Libertarians are utopians, and in their minds anyone who opposes them is standing in the way of utopia. They also have few if any principles or sense of ethics. Thus - even while pretending to support liberty for all - they're willing to lie and smear their opponents in an attempt to silence them.
But, if I had to choose just one, the main reason I oppose libertarians is due to immigration. Almost all libertarians are very bad on immigration, with some supporting literal open borders. The entries below detail how libertarian ideas on immigration would not work and would have a highly negative impact on almost everyone in the U.S.