Big Mom was plenty terrifying when she fought.
Bro....
This is the culmination of her character in the Wano arc to sum it all up. An amnesia storyline, and her unknowingly aiding the Wano rebellion with every action she makes. Not only does Wano have lackluster fights, she contributes to it by outright
removing them by taking out two much needed matchups for Nami and Usopp, who lacked any of them post-ts.
So basically you find a character that is an Emperor of the sea who was the previous main villain of an arc who undergoes a universally criticized amnesia trope, adding nothing to her alliance with Kaido and instead causing him trouble by knocking out his subordinates which removes potential matchups for Straw Hats, and for her to conclude her role here by getting defeated by 2nd rate Supernova like Kid and Law who she had no connection to prior, to be terrifying. This is what you are saying, no ifs or buts.
The Scabbards got there moments in, but they were never going to be top players fighting Kaido? Like who even expected that?
Who said anything about Kaido?
I'll try to explain this as simple as possible. The scabbards are definitively the emotion centerpiece of the Wano arc second only to Momonosuke, which means they play a role that previously established characters fulfilled in previous arcs. This would include people like Vivi, Rebecca/Kyros, and Pudding.
Let's use Vivi as the example since she is the most popular and easiest to use. Throughout the Alabasta saga, starting at Whiskey Peak, we as the readers get to know her struggles and pain for her country as she starts acclimating to the Straw Hat crew. She represents the suffering that the Alabasta people go through, which makes us care about her as a character. And what we got in the Alabasta arc did not disappoint, as she played a very active role in trying to save her kingdom. She was given plenty of moments that are memorable even to this day. Vivi is an example of how Oda uses arc/saga specific side characters to enhance the current conflict that is going on.
Now, to the scabbards. As far as build-up and their introductory moments go, they are arguably done ever better than Vivi. The ''Raizo is safe'' moment speaks for itself. However, unlike Vivi, they did not deliver on their hype in the Wano arc. In particular, Act 3, which makes up roughly 80% of the arc. Their most contributory moment was stalling Kaido, but that was the extent of it. They were giving basically no screen time, and the ones that they actually get are seeing them cutting down fodder enemies. We even see Oda use convenient methods to take them out of the story, such as giving Kinemon and Kiku fake-out deaths, which lasted till the end of the raid. Denjiro was gone for 40 chapters, only to cut off Orochi's head, which most people already expected was to happen. Let's not even get into Kawamatsu...
It's not hard to imagine why the scabbards leave a lot to be desired. It's not about the fights, it's about the characters.
Not just the scabbards actually, I can point to several characters like even Kaido, who simply didn't live up to the expectations.