To me, the single biggest argument against LLMs and generative AI is this:
It is a technology whose sole purpose, by design, is to persuade humans to accept what it has produced, with no regard for correctness. Bottom line, that's mechanically how the technology works. An automated grifter, thief, liar, and manipulator.
And it's so disturbingly effective and in widespread use that our very sense of reality and truth under attack.