In Transformers Prime, Cybertronians are apparently vulnerable to extreme cold. (“You know prolonged exposure to sub-zero conditions can cause permanent system damage.” -Ratchet, S1 E7)
They are in danger of freezing when they go to the Arctic which continuously becomes a plot point, but somehow handle themselves just fine in space which is definitely way colder. Which leads me to believe it’s probably not the cold itself that’s dangerous to them, but the moisture in the air that freezes up around all their joints and crevices that immobilizes them.
The humidity in the Arctic is actually quite low, but in a snowstorm (like the one we see in S1 E7 where Optimus and Arcee nearly die) the snow that hits their bodies could totally melt from their body heat only to re-freeze around them as clear ice, expediting the process. It would gradually make it more and more difficult to move.
It’s like the clear icing that planes encounter when they fly through moist air, where supercooled water droplets freeze on their surfaces and make it difficult to move the control surfaces on the wings. Which is why I imagine flyers are probably more resistant to the cold+moisture conditions, since they already have built-in anti icing. Starscream does complain about the cold a lot in S2 E13, but that might just be Starscream being Starscream, or just trying to manipulate the others into letting their guard down. Dreadwing and the flying vehicons don’t appear very bothered, but then again neither does Optimus, so that one is staying as a headcanon for now. Maybe Starscream is bothered because he doesn’t have paint for extra insulation.
Optimus does say “the current temperatures are not extreme enough to affect our biology.” during that episode, and when he does there’s only a little bit of snow, which might confirm that when they say “temperature” they actually mean “moisture”. I don’t know.
The fact that cybertronians even have the ability to “scan” vehicles (created by other alien species) tells me that they’ve always not only been aware of other intelligent species, but also possibly interacted with them. They were designed to interact with them that way, assuming scanning was an ability Primus gave them and not an extra gadget they added into their frames afterwards (Which is also an interesting concept) But maybe that interaction only went one way.
Taking the catchphrase Robots In Disguise™️ seriously, it is easy to imagine cybertronians being a horror story for other species. Aliens that hide in plain sight, lying in wait. Indistinguishable from the very vehicles you use to travel around. Would you trust your car, be sure it wasn’t replaced? Cybertronians would be TERRIFYING if they wanted to conquer your planet from the shadows.
On another note, I like to imagine there’s More Than Meets The Eye™️ to scanning altmodes. It’s not just to hide. Take TFP for an example. Neither Airachnid nor Dreadwing/Skyquake transformed before scanning an altmode even though they were in situations where doing so would have greatly helped them. This was probably an animation thing, the people making the show not having the time and budget to design two seperate altmodes for the characters.
However, if we read into it, I think it’d make sense that the true reason they scan earth-based altmodes is that their cybertronian altmodes are not fully compatible with earth. Or at least they don’t know how compatible they would be so instead of risking they wait to scan an earth-based form. Especially when it comes to flyers. The atmosphere, the gases in it, the gravity, all work a whole lot differently on other planets. It’s more advantageous to become something that was built to maneuver on this planet, than trying your chances with your cybertronian altmode and seeing how it fares in this unfamiliar environment, while also in the middle of a battle.
The Autobots specifically mention to Wheeljack and Smokescreen that they needed an earth-based altmode for disguise purposes, so we know they could have probably transformed this entire time but simply chose not to out of convenience.
They would probably not be able to go back to their earth-based altmodes again without scanning if they reverted to their cybertronian ones, which is why they don’t when they go to Cybertron. I also imagine the process of un-scanning an altmode (dunno what else to call that) would take a while, so if it ain’t broke don’t fix it.
In episode where Arcee and Jack go to Cybertron, she’s gonna come back in a few hours so why would she go through the trouble? In the movie, they don’t really have a reason/motivation to do so. The Autobots probably want a reminder of Earth, Decepticons like Shockwave and Megatron already have cybertronian altmodes. I don’t think we see Knockout transform but he’s always been very vocal about how pretty he is as a car so he probably kept his altmode as well.
That leaves only Starscream to be explained, and my personal headcanon is that he secretly grew to like human-made aircraft. Although it would’ve been cool to see him as a cybertronian jet in the movie, the movie scriptwriters didn’t really care about Starscream anyway.
In my AU the earthplane enjoyment is in part due to him bonding with Nicole and pitying all the trouble humans have to go through to achieve what comes to him naturally.
