Because of the recent release of Deltarune's second chapter, I've been thinking about the power of Determination from Undertale and my headcanon/theory is that it has a weakness. the beginning of Deltarune has you agree that "You will accept everything that will happen from now on" while in Undertale:
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Chara will offer you a contract to get back the world after a genocide run.

Why are these two so significant? Because I think it has to do with Determination. Determination in that universe is the power to never give up, to keep going to incredible extremes. So when they do something, they keep doing it no matter what happens. If they make a promise, they will not go back on it. A contract or agreement is basically just a two-way promise that you hold to. Meaning the weakness of Determination is.....that it can't or won't break any agreements or contracts it makes, because it can't give up on holding itself to it even if makes no sense. This explains why the player don't have much effect on the story of Deltarune, because they agreed to accept that it has only one ending, limiting their determination from changing it. While in Undertale
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you can't get your soul back because you agreed to give it away to Chara. the player can't take it back with the power of Determination, because that would be going back on their agreement.


Meaning a human with Determination is much like a fair folk that can be trapped in a contract if your clever enough: if you get them to make the right agreement that you can exploit, you win.