Flaws and mistake Decepticons make.

Underestimating humans.

Walking head-on into artillery fire instead of drawing back or getting to cover.

Leaving evidence of their assassinations.

Taking prisoners instead of faces, and taking prisoners when there's no leverage to be applied from them.

Revealing themselves to the world and then not following through on that intimidation.

Splitting their already miniscule forces.

Working in small infiltration teams instead of decisive strikes.

Relying on eldritch Autobot artifacts and experimental macguffins instead of good old fashioned planning.

Ha ha, being in a Micheal Bay film. The guy is fantastic at explosions and action but he just doesn't do well with movie villains.

Being in a big budget Hollywood summer movie. Hollywood has gotten it into their heads that the only ending people will accept any more is that the good guys win.

The plans being too big. Everyone knows that Hollywood loves to milk franchises to death so if your plan as the villain means the destruction of Earth or some other way that will end the franchise then your plan is doomed from the first frame of the film.

Having the worst armor of almost any villainous movie robots really doesn't help them out either. At least Battle Droids can say the Jedi had a light saber.

Read the novels. The Decepticons are more competent.

The Cons' biggest mistake? Not adapting. After the sabot round fiasco, for instance, they ought to have developed resistant armour.

Why haven't people developed bullet-proof skin?

I have bullet-proof skin. It's smooth but itchy and smells of strawberries.

But if you mean the Decepticons can't adapt their armour because humans can't adapt their skin then that's somewhat incorrect. Humans don't go into battle naked (anymore). And Decepticons are alien beings that can change their bodies at will but just don't seem to have bothered doing much of anything to adapt to the weapons that have been used against them since they arrived on Earth. Currently humans can't change their actual skin to being bullet-proof. Maybe in the future we'll be able to though. But what humans can and cannot currently do has no bearing on why Decepticons can or cannot adapt to better protect themselves or be more effective.

I'n guessing though your comment is probably more just a snark at people having a problem with weak BayFormer Decepticons.

Someone call Overlord Balder.

They should've killed the bots to begin with. Also when did they ever leave evidence of their kills?

Being the bad guys in a Michael Bay film.

Originally Posted by Meta777 One day, he will return. Yes, he will return, and we all of us shall behold the ultimate glory of Aggron Archer!

I was going to say the exact same thing!
Seriously, what kind of alien, robotic species is built out of paper mache' and tin foil? LOL!