"These are not the droids you are looking for"
Yeah, Star Wars security was top notch.
Seriously though. the thing with the MCU is that it really depends when it happens. If you sent Sidious back in the 40s, for example, I think he'd have been able to take over without too much trouble. I think people in the MCU are a lot more alert to these kinds of threats now, so he will have a much harder time taking control of anything.