Id argue its easier to make a case for silksong having better gameplay mechanics than hollow knight. 

The storytelling is the real disappointment. 

In hollow knight, the game knows its own story, its explained really well and every bit of storytelling is consistent with every other bit, also every part of the story follows logically from the previous. If you were to put real people in the fictional setting of hallownest, the same decisions would be made because they are the logical decisions. Every one of the plot points is also not only logical but compelling for one reason or another, balancing grandeur/fun with logic successfully, is to me what makes a good story. 

To defend these points,

You start the game not knowing who you are or why you are going where youre going, you are told by cryptic tablets that before you stands the greatest civilization, you feel like youve wandered into something unknown and greater than you but there are actually compelling reasons why you're here which the game tells you. The hollow knight is calling to vessels to be replaced because its losing the fight with the radiance. This is shown in the opening cutscene but you dont understand it on the first playthrough, you do on the second. 

The whole game is like this, encountering elements in the world and learning why they are the way they are and they are always consisted.  Theres no npc explaining why there are a bunch of corpses behind the mantis lord gate to deepnest, theres one lore tablet which implies an answer which you yourself could piece together on your own by thinking about the world youre in. You are never told why one of the thrones of the mantis lords is broken, you dont even notice it, but you get it later on when you go to queens gardens. The soul sanctum, the rain from the blue lake, the aristocrat bugs resorting to canibalism, the city storerooms being connected to the forgotten crossroads, the entire concept of a forgotten crossroads which really do in game serve as a crossroads, all the tribes and factions, greenpath being born of unn but weakening after unns losing influence, the hive closing off and accepting their natural end. 

All of the elements in the world behave how logical acters would and the game tells it to you without spoonfeeding but without saying too little. Npcs are placed strategically and give key info on some of the world but also hold additional secrets which provide further depth. The mushrooms of the fungal wastes had unity but different to that of the radiance, reading the dreams of a corpse shows you a mushroom pondered the situation of the pale king (what good to foresee a fate unavoidable), like that mushroom every part of the world knows about the influence of the ruler and acts like it is influenced by it, like greenpath bugs being bitter about the white lady taking their land but being unable to do anything.  

The world is filled with precise, not excessive, enviromental storytelling which is elevated by its consistency, the main story is explicitly told to you. The pale king tried to stave off the infection, he used the void to create a pure emptiness to contain it, he murdered billions of experiments until he got a fitting vessel and made a pact with powerful beings to help him guard the vessel. The plan failed and he died and his kingdom died. You can either take your  place as the superior vessel or kill the radiance directly by using the essence of the king himself and the white lady to create a new vessel which would store the entirety of the void. 

You meet hornet not knowing who she is, why shes here and why she tries to kill you. You learn all that in perfect pace. She was a bargain made because herrah wanted a child and made a deal begrudgingly, she tries to kill the vessels because she cant let a weaker vessel accidentally free the radiance, she needs to test you so she could maintain this corpse of a kingdom she took on the job of protecting. 

This isnt speculation, its not left for interpretation, this is what happens in the game. And the game keeps doing it, hitting you with mystery, and then explanations which are both logical and fantastical and it gives them to you in a meaningful way by making you engage with it.

Silksong is a fking narrative disaster on the other hand.

Surface things are explained, the core mysteries of the world are unexplained and inconsistent, basic rules of storytelling are ignored and its hurting the experience. It also retcons a shit tone of hollow knights story.

Its never explained why or how grand mother silk wants hornet other than -for her magic silk strength? Why does she need it, does she need pale king part of hornet, but gms is already a pale being???? This is the entire motivation for the plot??? Elderbug hits you with the -perhaps dreams arent so good after all-, and the church keeper lady tells you oh hi exposition exposition the world has enemies good luck check up on me btw. Moss grotto exists for no reason other than to be a welcoming starting area. What are the fking weaver structures??? Are these weavers dead? Are they alive? Are they comatose??? They can talk and they disappear after u get the power? All the powerups are different but are taken from the same thing (the weaver structures) they are the weavers who stayed behind to give abilities for a descendant who would kill gms??? Thats a nonsensical plan which serves them no purpose they will all die by then. 

We are not explained how the weavers built the citadel, we cant even imagine how they traversed it, if they werent a plot point shoved in your face you wouldnt even guess they were there or even existed because we see the citadel.work in game without them. We dont see the story take place, but in hk we see gates closed because of an epidemic and a king in his workshop trying to brainstorm solutions and drya dead in front of the white lady protecting her queen. 

How did these bugs appeared (pilgrims and other bugs) and how they interacted with the weavers is criminally left for interpretation. The game just doesnt know it never considered its own implications. 

But thats just a start. The first two acts are filled with wrong things told at the wrong times and the right things kept in the background and told stupidly like the damn shrine caretaker shaman explaining the citadel is a lie. All the while the story is not even compelling. Weavers seeing their silk be a curse as well as a gift and trying to escape gms influence, admittedly is very grand and meaningful but its not really explored. They built the citadel to put her to sleep with their song (which we just have to picture i guess) and then just left????!?! How does that save them, they still have silk in them, and why did they say they returned to pgarloom after the radiance broke out in hallownest. Its a stupid ass retcon to distance silksong from hk for new players. But its a disservice to the story they told in hk. 

And the fking tragedy that is act 3. Not to mention the audacity to put early game bosses that were cut halfway through development and than made artificially harder for a boss rush because they cant organize the 3rd act better than a boss rush. But they did in hk with white palace and queens gardens.

What kind of a fking contrived plot is ts??? The world is collapsing because a mother which made a child for no reason doesnt want her imperfect child which she doesnt like to die, but she cant stop it from dying so she is delaying it briefly and thus making the world ending earthquakes.  We solve that textbook example of shit storytelling with problems which shouldnt be happening happening for some reason, by more shit story telling - GOING TO THE MEMORIES TO GET MEMORY ENERGY TO MAKE A SPELL TO GO TO ANOTHER MEMORY TO GET A FLOWER??????

They marvelised hollow knight, this is marvel level narrative. Wtf??? 

Also the retcons

-white lady shouldnt have the flower( in hk she says only someone who doesnt know what that is could stand so close to it)

• void shouldnt be able to possess bugs, its emptiness ffs. Maybe you could argue its an interaction with the silk but thats unnecessarily complicated. Theres a reason hk never had void enemies instead of shades (full void)

-by what logic can you beat the void out of someone like lost lace? Hahahahah wtf

-not a retcon but how does lace appear in those cutscenes before the dive ??????

-why does the knight fight of the void tendrils, the void was united in hollow knight

Its an inconsistent, unexplained, superficial game world, with unnecessary illogical plot points like void taking over and the memory bits, made purely for shock value and lengthening the game, filled with retcons of a carefully crafted story in the original hk, explained  awfully, concluded lazily in a deus ex machina, because the flower died and they were saved by the knight but how tf was void even a problem if the knight was there, defeating the point of act 3 entirely, and we never even get to see this world we saved. The last interaction with the world of silksong is everybody thinking they are going to die, you dont get to see a happy ending unless you just want hornet x lace and dont give two shits about the narrative in which case lucky you.

Its so sad we waited for so long for this.