Some example reading to open your mind: https://mises.org/library/externalities-argument
Argument that "it's hard" is not much of an argument at all. It does make me think that there's considerable flexibility in a "libertarian" position on externalities though.
But I do agree that the best pricing schemes involve settling on price through transaction. Take, for example, cap & trade schemes. This is a widely used mechanism to manage negative externalities.
Externalities exist, whether or not the "Austrian" school has a way to deal with them. Considering the issue, deciding that one's philosophy can't deal with them, and therefore deciding to stick one's head in the sand instead is why mises.org remains fringe. It's an irrational, religious framework.
Read Mises Human Action and you will understand.. Austrian economics is a priori (built from first principles) not a posteriori (empirically derived)
Fine it's impossible to correctly calculate externalities. I don't see the line from there to: "BigCo dumped a ton of waste in my pond, but it's unfair for the government coerce BigCo to compensate me because there's a chance the I might get more than the true value of that pond."