Because of sweeping attack, people have accidentally killed their pets or died by hurting a neutral mob, like an iron golen. Because of the sweeping attack, I have died before because i try to hit something else but I accidentally hit an enderman, and I died from it. Earlier today, I tried to hit a boat with my cat in it, but the cat died as I hit the boat. Here's a bug related to boats and the same issue: MC-169637.
Not only that, but tool cooldown is also a bug with 1.9. This makes it harder to kill things. There's a video on Youtube about the top 5 worst minecraft updates: https://www.youtube.com/watch?v=ER2vEC9lMac&t=1s
and 1.9 is on that list. Other than the sweeping attack, and the tool cooldown, 1.9 is a good update. Sweeping attack itself isn't that bad, but the fact that sweeping attack can hurt or kill your pets is an issue.
Both of these are litterally not bugs but features...