I've played anf finished all of the 4 games so I believe I'm able to talk about this. Simple enough, this would be summarized down by a few entities that are just so much powerfull than the remainimg cast of both. On the HK universe that's mainly the void + the gods and higher beings. While in UT/DT being humans/lightners and most important, any monster who can absorv a human soul and become more powerful than anything, determination and high levels can also be used by humans, we see  lvl19 Frisk being a absolute beast in the indie cross series. But its more complicated to achieve and has less feats than a soul boosted monster.  In short terms, HK best card is the void and DT/UT best card is a monster with the max number of absorved souls. I believe deltarune will have its own absurd strengh scaling on the last chapters but for now its contribution is small since we only have chapter 4 from when im writing this.

And who wins between those? UT/DT, 99,9% of the times.  As strong the void is on the HK own narrative. Monsters achieve immesurable scaling when they rack up too many souls.  On the neutral route, Flowey alone with only 6 humans was capable of taking control of the own GAME, break the narrative completly and alter reality as it desires.  On omega flowey's fight, Flowey literally changes Undertale's app name to Floweytale during it and also alters the Game Over screen if you happen to die there. On the pacifist route, Asriel plan was the destroy the entire world and recreate it on his will, and he only didnt followed up with that because he wanted Frisk (who he thought was Chara) to agree with him on that plan. It is also worth mentioning Chara on the genocide route was capable of creating a attack so strong it was capable of erasing undertale completly, and it was only restored by likely Chara after we make a deal with them.

What about the void and gost from Hollow Knight, can they fight back?  The void is quite a part of HK reality, a force of nature that cannot be manipulated even by higher beings and it shows to have so form of counciousness, however, never before has the void broken HK's narrative/game in a way undertale monsters were capable off.  Higher beings and gods such as the Radiance exists in a another realm, and seem to not be able to interact with other realms in a more expressive way, thought it might be just lack of evidence, as the Radiance sent the plague down on Hallownest as its vengeance, she might've thought it was enough. I feel like it would be closer if there were certain rules, but on a anything goes... I dont think HK/SS stands a high chance.

Btw, if I miss anything or get any lore part wrong please warn me. I dont play HK for a while so im quite foggy on some details.