Specifically Abathur and it all comes down to how Yuumi carries no consequences for 90% of the game.
Abathur for me is a very special Hero within HoTS he is one of the first characters I played and one of the biggest archetypes I wanted to see explored in League and oh boy how I regret it. But all of that mainly comes down to the execution of Yuumi's kit.
The main issue I've seen people have with Yuumi is that she's too safe as she can basically hop around champion to champion without ever being touche unless she's interrupted mid jump and despite having a kit that's very similar to Abathur, he's never seen this issue because of one big change. Abathur himself never actually sits on other heroes, instead he finds a safe spot on the map to turn himself into a cocoon from which he controls a monster that actually sits on the heroes.
This means that you can easily try to find Abathur's cocoon and simply kill him if he doesn't pay enough attention to his surrounding. Invading heroes could try to track him down and Abathur himself needs to constantly be aware of the enemies presence on the map and be sure to move accordingly to always remain in safe locations.
If Yuumi had a mechanic similar to this, where she's still not 100% safe when simply sitting on a fed enough champion, I feel like her kit in itself will feel a whole lot less unfair in return.
This is in no way me requesting for Yuumi to be changed to an Abathur clone but rather a starting point for a discussion and a comparison of the two as a means of reasoning as to why I think Abathur's kit was implimented well for HoTS while Yuumi's was not for League, or atleast in my opinion.