This is a question regarding the special ability of the Hierophant "Sway the Masses". One of my players has just started with a character like this and we think that it seems odd that any, absolutely any, roll can be succeeded with. So I've come here to ask if other GMs have implemented some form of limitations for when this can be used?

The reason we ask is that we think that it could be broken if the character can just walk up to the planetary governor and take control over the whole planet to bring the whole PDF down on the heretics, and then use it again to escape any and all consequences for his actions, whatever they might have been. Or for that matter go up to a Chaos Champion, or Daemon, and simply charm him into attacking his own cult during the climatic shown-down, or command the cult to kill its own leader or whatelse my creative players can make up in their minds. I know that they can find many ways to dance around pretty much anything with something like this.

Or is it meant to be this powerful and it evens out in the long-term?

You have to keep in mind that social encounters are not solved by a single test. It works (as written in the book) with disposition values (Page 277 Core Rule Book). An NPC has a value and a personality. Depending on the personality different social skills increase or decrease the disposition. You as a GM can test on the dispostion to see if the PCs got what they wanted from the NPC. This way they have to waste a lot of Fate Points to ensure success. If they do that I would allow it, but would make sure that they get into a situation later in which they really would have needed the fatepoints.

For the example with the Chaos Champion or Daemon or Cult: Why should he/they even listen to the characters? Is it intended that he/they talk(s) to the players?

If yes: If they want to spend a fate point that way, let them.

If not: You can't convince someone who isn't listening to you or who is does not follow the same reasoning as you.

A combination of multiple rolls per social encounter and a reasonable limit on what can be rolled for (and thus auto-passed) should limit any potential abuses. I haven't used the disposition rules, but no significant conversation (such as one with a planetary governor) should come down to a single roll. Likewise, unreasonable requests ("Hey why don't you give me personal control of the planet's armed forces? kthnx") shouldn't be tested for - they shouldn't be possible.

As the GM, you decide what success and failure means. So even if your player is auto-passing tests, what that means in terms of the narrative is still up to you ("No, you can't have control of the military. But you can have a squad of Arbites for a week to help with your investigation")