So Iv played HD and RT, DMed a bit of both.
Now Im looking into BC, but I could do with some info:

Whats the scale?
Are we talking a group getting things done like DH, or small armys, huge armys, Rogue Trader level or beyond?

Length of Campaigns?
Is this the sort of game that you can do one offs or campaigns that only last a max of 6 weeks? assuming you play once a week for a few hours?

Is the an inbuilt leash?
I One of the biggest things stopping me from DMing this the thought that players will see this as an excuse to just go nutty. Im assuming the dark gods have some sort of guillotine hanging over their heads in case they get out of hand?

Thanks ^^

The schale could be whatever you feel appropriate - from lowliest cultists trying to survive while exploring their new faith, to daemon princes leading the Black Crusade. It is possible (and intended) to change schale during the campaignas heretics gather infamy and power.

Length of adventure is not specified, but do not expect to reach endgame (apotheosis/leading a crusade) in 5-6 sessions.

The acts which can anger the dark gods are indeed specified and are intuitive (khornate showing cowardice, tzeenchian getting fooled by someone etc.), but the main factor is warband rivalry. Lots of champions want to achieve glorious apotheosis, few reach there, strong warbands subdue and pillage weak ones, betrayal and conflict are common - and that's only internal enemies we're talking about.

The assumed scale is easily on the level of Rogue Trader, potentially bigger depending on your approach, but it can relatively easily be scaled downwards to a degree, depending on what kind of campaign you want to run. But a lot of the rules will assume that you are fairly well-established heretics with a power base.

This isn't always made overtly clear, which is why I'm telling you, because some rules will feel awkward if this is not how you start out your adventure. Rules go on about servants and grand destinies, the acquisition system is very abstracted based on this, the rules for making your own compacts (which are.. questionable when it comes to the published adventures in how it's supposed to work, or how it's supposed to work from an in-character type of play) specifically mention pulling on largely abstracted resources, etc.

Which is all kinda funny because the free starting adventure does the completely opposite - it strands you as destitutte heretics out of your element and time.

So the scale is a bit messed up at times. It's as if the various writers couldn't really agree on things or wrote things without knowing what was going on arounds them. I don't know.

Campaigns are entirely up to you. The published adventures can take anything from a single session (which I have never been able to do, but it's up to your playstyle) to a couple of sessions (3-4) depending on how you run it. The rules for (custom) Compacts are very open-ended, letting you and your players determine what you're doing, if you use the system (my group generally doesn't, because it feels very artificial and we're having a hard time making sense of it; Compacts are presented as some kind of default assumption, but also makes assumptions that is in no way true for a lot of games).

"Ending" the game can take an enormous amount of time, depending on how you play and how much Corruption/Infamy you reward.

And if the players go nutty, let them, just remember to remind them that heretics or not, there is always someone stronger than them. Like Chaplain says above, the acts which can anger the dark gods are specified and fairly intuative, which means that you can quickly start determining that for yourself. I don't even consult the rulebook anymore, it is really about pleasing the dark gods in general, or pleasing your god specifically, or failing them spectacilurly in general, to gain various degrees of Corruption - the double-edged sword that will either give great reward or great punishment, depending on their fickle nature.

Infamy is all about doing things that makes you infamous with the forces of Chaos, whether burning down a church or killing a champion of the Imperium. Making a name for yourself. Intuative.

I like to use the game of thrones analogy. Sure they can start to go nuts, but then someone else just as powerful will take exception and put a stop to it.

As for control, you can always hand out corruption for your players being stupid.