I think that's why she generates such hate. It's the old League anti-fun shit. Getting killed while stunned is a hundred times worse than getting killed while mobile. She has a negative fun-factor for enemies. Infuriating to die to, stressful to beat unless you pop her from long range.
She's also getting more common now that people have realized how hard she counters Reinhardt pushes especially.
She was originally the hero I saw the least, now it's hands-down Zarya. Zarya's actually pretty cool, I think she's just confusing and her utility isn't obvious. Her damage without several good blocks is negligible but she hits like a truck with maximum charge. She's also one of the tankiest heroes straight-up damage to damage, and when you add in the shields I rack up 0-2 death games all the time on her.
The other thing is the need for coordination. SHield only lasts two seconds, and most randos only realize they've been shielded 1.5 seconds into it. Same thing with the ult. Everything she does is team-dependent and you can't rely on your team to combo with you so you have to follow them around and dance to their tune instead.