That is a little overly simplified. Sure there are lots of haters that fit your description of disliking losing and failing to adapt. But this is not the core issue. The core issue with Shaman is that she is indeed fundamentally overpowered. She has speed, power, an extreme very variable and viable kit, huge distance that she can cover with one leap, running speed, throw distance, excellent recovery times and last but not least move cancel options like no other.
She can soley abuse how inferior other characters are just by feinting wild cat's rage, predator's hunger/mercy over and over again and wait until a dodge happens. Since her pounce or neckbite comes out so fast, you don't have any possibiltiy to get out of recovery to attack her out of it or dodge again. While she can double dodge for instance a highlanders fumorion kick cancel to caber toss. This all is just an unfair advantage which gives the shaman complete control over the fight. For other classes this is always an uphill battle. Possible but as you said you need to be careful and the shaman player to fall for it.
Thx to her as you say fluit kit and her great recovery times she can easily outmanouver other casts members that feel much more clumpsy and less reactable. It makes her easy to use yes, but I would say none the less that Shaman requires some knowlegde over her kit and how it works together. But once that is figured out then you are in trouble.
But I would still say that In the hands of a novice who just started the character and just tries to get a hang how things work shaman can be dealt with quite easily. Which was also the case for many players on the opposite side as she was released. Unlike Centurion on release who brought an entirely new fighting challenge to the game and promoted to react at different and seemingly everchanging timings combined with unusal tracking + stamina draining options than ever before + getting severely punished with a cutscene when they screwed up. Shaman's only cutscene is her bite she promotes actual fighting and a dynamic fight - but she is just far superior to the other characters in mechanical terms while having advanced versions of some of the best/unique moves in the game.
Some examples:
Wild Cat's Rage (Jump attack) aka Orochi's Storm rush advanced
- Way too fast. This single move ensures that the Shaman always gets a punishment. If there is any kind of opening she can easily benifit on it with this attack. Often it was stated on reddit that the indicator is bugged and hence you have an even shorter reaction window to actually react to. I am not sure if this is fixed already but I usually try to predict where my opponent's gonna hit and concentrate on blocking to maybe be able to swap directions at the last moment if I have to.
- Covers way too much distance. This soley guarantees that Shaman is superior in every 4v4 game mode (especially in tribute) She can be everywhere on the map and engage opponents. If she is right on the edge of your screen not 1 second later she is in your face probably dealt some damage already and go into her unblockable mixup. Furthermore I cannot stress enough how insanely powerful this is when it comes to team fights. She can just instantaniously disengage an opponent and jump upon another maybe to interrupt a lethal attack on a team mate without actually having to mind much about her own opponent and punishment possibilities.
- Can change directions very shortly before the actual attack
Her throwing distance is also as mentioned before unnecessarily high. That only gives a huge advantage in 4v4s but also in duel situations as you easily wallsplat.
Her side dodge attack Wild Cat's Swiftness is safe on block and is a heavy. Meaning good damage while having no thread of getting punish as severe as a light parry punish. Furthermore it is short ranged/quick. Meaning opponents don't have much time to prepare her to punish with a parry nor to readjust if they feinted.
I don't know how the reworks turn out to be, but surely there still have to be some readjustments to be done to the shaman to get her to a reasonable spot. The changes so far are going in the right direction (for instance the increased recovery time after a missed predator's mercy) but it definitely should not stop there.