@Communist Starlight
A combination of things: for one, I think the show needed another season of two at least to tie up all the threads they were weaving, such as the issue with the Lothal Temple, stopping the TIE Defender production, and the liberation of Lothal from the Empire that having it all resolved in the final two episodes makes it very crowded.
Related, having Ezra face and best both the Emperor and Thrawn in the last episode
really borders on Fanfiction tier writing when you step back and think about it. You’re facing both the ultimate big bad in the OT as well as one of the most well known and respected villains from the EU. Beating one or the other, much less both in the same episode, is feat that requires some very skilled writing that I don’t think the writers pulled off. Some people might like it, but I think the reintroduction of the Space Whales was way to deus ex machina.
And that bring us to Thrawn - while I enjoyed him immensely in season 3, I think he was misused and ultimately wasted in season 4. Personally, I think Governor Pryce should have been the main antagonist for that season, and she should have been given a bit more depth and complexity to her character, since that had already been hinted at in the novels. I don’t know, maybe I’ve just been spoiled by Mobile Suit Gundam that I want my antagonists to have more depth and characterization.
This also goes without saying that from I’ve been seeing, the final episodes of Clone Wars completely blows the final episodes of Rebels out of the water, so I know the creative team can and could have done better.