Thrawn in Rebels has all the resources of the Empire and he and his underlings still manage to lose regularly to a far weaker rebel group. So it’s not really the same character to me.
When Thrawn loses in Rebels he always plays the ‘we lost the battle but not the war’ card. It’s the same deal in Heir to the Empire. If I remember correctly he loses his first big attack on the shipyards. He scores some big victories in the following 2 books but that’s the same case as in Rebels - at Atollon he destroys most of their fleet and destroys the entire strike force attacking his Tie Defender factory.