What is the main criticism of classical economics?

Recently been reading economics in one lesson by henry hazlitt and the main point he makes is that most government spending is bad because bad that money been spent privately it would've been utlized more effeciently and met peoples needs more.

He also says that its easy to think that gov spend is useful if you dont think of what could've been there had that money not been taxed.

He at one point mentions the long and short run and why we should prioritize the former.

Till now it all sounds logical, so what are the main criticisms of neoclassical/austrian economics?