For starters, there's the theme.
She’s a Siege Tank.
Siege Tanks aren’t supposed to be durable, have a ton of self-peel, or be very mobile.
Hammer can facetank Chromie’s burst with Neosteel Plating, peel divers off herself with Concussive Blast, and then zoom away with Thrusters if all of that wasn’t enough.
Siege Tanks are supposed to have punishing AoE damage output in Siege Mode.
Hammer’s AoE damage output in Siege Mode is piddly, her splash damage is just 25% of her basic attack damage in Siege Mode.
Siege Tanks in Siege Mode are supposed to hit slow but hard.
Hammer hits just as fast in Siege Mode as in Tank Mode and only a little bit harder.
Then there's how people play around her.
Enemies who can ambush, outrange, or dive Siege Tanks are supposed to counter them, but none of those really counter Hammer in this game due to her durability, self-peel, and mobility. She doesn’t really have any dedicated counters because she can counter the heroes who should be her hard counters.
However, because Hammer lacks the range and punishing damage of a real Siege Tank, she’s not that hard to soft-counter. Even though the number of heroes who can consistently lock her down long enough to kill her is in the single digits, a significant portion of the roster can force her out of position if she isn’t guarded by her teammates.
That’s where the whole “protect-the-Hammer minigame” thing comes in, you need to protect her or she’ll basically be nothing more than a 5th health bar. And not a lot of people like feeling like they’re stuck on a leash to make sure their Hammer gets to be useful. I know I certainly don’t like playing “eat the Ming Orbs for Hammer,” even if I’m the Tank.
Finally, there's how she feels to play.
As someone who loved pre-rework Hammer, she is incredibly boring right now.
Pre-rework, her range and damage output was frankly terrifying. You couldn’t just expect to walk up to her like you can now (twice, since she’ll just knock you back with CB).
However, she was much more vulnerable to divers, ambushes, or ranged burst than she is now. She had dedicated counters like ETC, Diablo, and Illidan. Late game she could bully a Chromie in a 1v1, but that’s more because Chromie has never been a good 1v1 hero – in teamfights, Chromie was far more effective against pre-rework Hammer than she was against post-rework Hammer.
All that together made it feel like Hammer was both impactful and required some modicum of skill to play. You didn’t have 3 “get out of jail free” buttons since 1 of them didn’t even exist and the other 2 weren’t as powerful. Your positioning mattered a ton, as did your target-priority, but if you played properly you felt unstoppable.
Long story short, I think Hammer’s rework took her in exactly the opposite direction it should have. The only way to fix that is with another rework.