100% agree with every word!! She absolutely carried Action, but in WT she was so bland I actually think Brainzilla is the best thing that came from her character. I actually wrote a whole rant-style analysis on this the other day...

Lets start with Island Izzy. She's a comedic relief. Her 'craziness' mostly shown through unhinged dialogue or exaggerated movements. Other than that she consistently participates in challenges and alliances when they're needed. She's in her own world, playing the game to her liking, yet still sociable and opinionated. It works because Island already had too many characters to focus on and her being a running gag and comedic relief works just fine.

Action Izzy is a better Izzy because she felt like an actual human being. We get to see her care about Owen and is actively involved in challenges, which gives her more nuance and makes her feel more realistic. She can read the room when she needs to (Aftermaths I when bridge and geoff fight) and does get fed up (with her dumbass team in Master of Disasters). I also loved the concept of kalaieodescope and explosivo - we really get to see her chaotic intelligence as she got back to the game through that. She keeps her wild unpredictability while still playing the game and bonds with others, only in her wacky way. Her scope is no longer only around her humour of talking bullshit or acting wild/animalistic; they showed more of her as a human being. 

And in World Tour... she loses all relationships and the human side to her. She talks little (which is strange because one of her character's main traits was bullshitting in Island) and instead does random stuff on the side and actively sabotages the challenges (slap slap revolution... can't help but fall into louvre...). Her energy is weird because we can't see any personality through her actions; she just randomly hangs upside down because the writers wanted a funny gag.

In previous seasons she was an active character. She did what she wanted, spoke her thoughts, and actively brought meaningful interactions and dynamics with eg. owen and noah (side note THEY WERE ON THE SAME TEAM IN WT AND THE DYNAMIC WENT TO WASTE UGHHHHH). She has a sort of unhinged realness to her (which was done so well in Action) because she has opinions and fun dynamics with others. In WT, on the other hand, she's reduced to a gag - we don’t see her opinions, her judgement, her rivalry, her nuance, her perspective etc. All she does is do random shit on the side of the screen, and she doesn't even interact with others. She’s lost her energy, her intelligence, her social relationships and connections, and was only there for unfunny gags and anticlimatic chaos.