Apart from a hatred of Star Wars Rebels in general, I was particularly incensed by how Thrawn was outsmarted by some whiny kid in the series finale.
I don’t know how Thrawn could have expected the Rebels to show up with space whales biologically capable of jumping to hyperspace. That seems like something that would be outside of ANYONE’s calculation (and no viewer would have expected it either). More importantly, Ezra is able to effectively ‘communicate’ to the whales using the Force, and the point across Thrawn’s two seasons of Rebels is that the mysteries of the Force are outside of his calculations.
Thrawn isn’t infallible. He wasn’t in Legends and he isn’t in Canon - he needs to have at least one specific area of weakness otherwise he’s more or less boring. I admit, I wish Thrawn had done more actually smart tactical stuff in Rebels - there are only two moments in the show where I was impressed by something he’d accomplished or figured out. But he was never outsmarted.