I really do appreciate TLK finally introducing Unicron into the films, and for ending on some sort of hook. That said, I think they did a poor job of establishing Unicron and what his existence means in the context of the plot. Obviously they want to save most of him for the next film, but that doesn't do this film any favors in giving a clear explanation as to who he is. First off- there's not a ton of gravity to the actual reveal. When Quintessa says "Earth [...] has another name, Unicron", that should be a huge bomb being dropped. Instead, it's spoken somewhat nonchalantly against a wide shot- a close up of Quintessa delivering the line would have sold the gravity more. Optimus' reaction is also rather muted, and he simply describes Unicron as "Cybertron's ancient enemy". It's also shown as a wide shot with Optimus being electrocuted as it happens, so it's not really conveying any amount of dread or foreboding whatsoever. We as Transformers fans know that Unicron means business, but to the general audience and for the chronology of the film, it just feels weightless and an empty threat. I do like the idea of Pangaea's spreading out shifted the horns of Unicron across the Earth, and that Stonehenge is the center point ("maw"?). But we have no sense of why the Earth being Unicron is a bad thing, and what Unicron's presence means in terms of the WHY of robots constantly arriving on Earth (which is one of the central questions of the film). Nitro taunts Optimus that he "blew his chance to kill Unicron", and really, what's so bad about Quintessa's plan to drain/kill Unicron other than it also happening to be Earth? There's no indication of why the "dangerous secret buried deep inside the Earth" is actually dangerous. If anything, we needed a clear indication that stopping Quintessa's plan was a double-edged sword. Again, they likely want to save as much of Unicron as they can for the next movie, but just giving a decent picture of the stakes he represents would have done wonders and kept people hyped for the next chapter.