Several times through the movie, characters mention that Unicron is "trapped" in another galaxy. However, since he's eaten other worlds, he's obviously capable of interstellar travel. So why is he trapped? Well, Unicron mentions to Scourge that his hunger is becoming unbearable; he therefore must be too weak/low on fuel to travel across the much bigger void between galaxies. Of course, that begs a few more questions...
Why can't Unicron simply eat more planets to gain that energy? The answer to that is as simple as it is horrifying: before going after the Maximals' home, he must have already eaten every other energon-rich planet in that galaxy. He probably planned to used the transwarp key as a shortcut the entire time, and fully believed that Scourge could get it. His arrogance and gluttony proved to be his undoing, at least for now.
But if Unicron has already consumed that many worlds, where did all that energy go? Aside from the vast amounts of energy needed to accelerate something the size of a planet out of a star's gravitational field and course-correct it on the way to the next one, Unicron has been rather wasteful with his power. He granted some to his heralds- Scourge, Battletrap, and Nightbird (that we know of)- and also uses it to spawn a seemingly endless amount of Terrorcon drones. Conversion of energy to matter being what it is, it makes sense that Unicron would be rather low.
Optimus does better against Scourge in every fight, despite never training; using the same tactics; and never getting his initial injuries repaired (at least, not that we see). But of course, if Unicron is low on power, he may not be able to give as much to Scourge. It isn't that Optimus is getting stronger- his enemy is getting weaker.
He does use a slightly different approach everytime he encounters Scourge. The 2nd time they fight Prime resorts to just shooting him with his blaster and in the final battle he uses a splash of lava to distract Scourge.
Even so, Optimus would not have lasted long enough to claim victory without his team- Bumblebee freed him from the drones, while Mirage and Noah softened Scourge up. The theme of the movie is unity, after all.
At the end of Bumblebee, more Autobots are shown arriving to Earth than are featured in this film- where are they? We see with Wheeljack and Stratosphere that the Autobots are scattered across the globe. Stratosphere in particular may have been hiding in Ireland, if all of them adopt the local accents.
Mirage is shown to take on multiple vehicle forms simultaneously, unlike the other Autobots, and isn't shown to have multiple types of kibble like Shatter or Dropkick in the previous film. In fact, his robot mode is perhaps the most generic-looking of any Cybertronian we've seen, to the point where it's unclear what he transforms into aside from something with wheels and lights. In all likelihood, Mirage is a "Shifter"- a sub-race of Cybertronians descended from Amalgamous Prime who can turn into literally anything. Franchise lore even says that Amalgamous was a kind-hearted trickster, not unlike Mirage's own playful personality.
How is it that Primal could be named after the "legendary warrior" Optimus Prime if Optimus has not yet done anything of note? The transwarp key is said to be able to traverse space and time. Just like in Beast Wars, the Maximals are actually from the future, and travelled to Earth's past.
Indeed, Airazor even outright mentions at one point that the Maximals are from both the Autobots' past and future.
Optimus Primal also notes that Optimus Prime isn't the great hero he expected. . . implicitly because Optimus Prime hasn't yet become that great hero.
One for the previous film: In this movie it's established that a strong energon pulse is the only thing required to jump-start a Cybertronian's spark. About halfway through Bumblebee, the titular bot sticks his finger in an electrical socket, producing an energon pulse. This establishes that Cybertronian systems, when mixed with Earth's electrical systems, produce energon. That explains 1) why Charlie was able to resurrect Bee with the electric guns, and 2) how electricity actually works to incapacitate Cybertronians who aren't dead/dying- it overloads them with energon.
If the G.I Joe in this film are the same G.I Joe featured in Snake Eyes: G.I. Joe Origins it makes sense why they would be willing to enlist the Autobots help. Aside from the fact that the Autobots are a benevolent race dedicated to protecting humanity from threats like the Decepticons and Terrorcons, Snake Eyes: G.I. Joe Origins also establishes that Cobra has attempted to get their hands on objects of immense power such as the Jewel of The Sun and have probably sought things similar towards it in their war on the Joes. Having the Autobots on their side would be a major advantage in combating that.
Noah and Optimus Prime being Too Much Alike has a more meaningful context in a meta sense when you realize who was the inspiration for Optimus Prime's voice. Larry Cullen, an ex-captain of the US marines who served during The Vietnam War and the older brother of Peter Cullen. Not unlike Noah, an ex-military serviceman and the older brother of Chris.
How does G.I Joe know the Autobots are the good guys instead of lumping them in with the rest of the Terrorcons? Simple: Sector 7 informed them.
Burke admonishes Noah for using his real name over the airwaves shortly before revealing that he works for G.I. Joe, an organization known for assigning code names to their operatives.
Airazor mentions that other Maximals were living with her, but they all died between the time the Maximals divided and the present. Given that Earth is an energon-rich planet, they must not have died of natural causes. But the Maximals are shown to be tough even by Cybertronian standards- so what killed them? Humans seems unlikely, and the Terrorcons didn't come to Earth until the transwarp key lit a beacon. There must have been something else out there, something that Airazor didn't get a chance to tell the others about. And whatever it is, it may still be there.
Unless they all committed suicide, which is horrifying in its own right.
Infighting?
The Autobots have been trapped on Earth for seven years. That might not be a lot of time to a robot that's been alive for at least thousands, but its definitely enough time when there's a war going on for the circumstances of battle to change drastically. The Decepticons have had nearly a decade to do as they please on Cybertron (not counting any time the Autobots spent in transit). Even if Megatron is absent, the war is going to be even harder to win now than before.
Also there is the fact that in this new continuity the Autobots are considered a resistance of freedom fighter and right now are on the losing side of the war. In fact losing so bad that the whole reason they were stranded on Earth was that they could find a planet to regroup and build their strength before returning back to launch a counterattack. Now think of all the Autobots who didn’t have the luck of escaping. Without Optimus to lead them they are probably getting slaughtered in droves. No wonder Optimus is so desperate to return to Cybertron at the beginning of the film.
It's mentioned that Unicron possesses the very souls of his heralds. Given Scourge's appearance beneath his mask, and that Unicron's only interaction with planets is consuming them, it's likely that the Terrorcons are undead like in Transformers: Prime, and are simply plucked from the wreckage and enslaved. This would also explain their fanatical devotion to their dark god- Unicron is the only thing keeping them "alive".
