Having run this setting in the past on other sites I'd also like to add that given the sheer scope of the world, getting characters that meaningfully coexist quite interestingly can be...a pain. And while everyone is united in service to the God-Emperor in the RPs, usually...the details are often so disparate that except for the Emperor, there's rarely much that really binds people together. I find too that allowing inter-departmental characters and interactions cause problems too unless there's a very good reason to throw everyone together and a very tightly-controlled narrative. There's only so much that an Mech-Priest and a Commissar can realistically interact about unless both are written exceedingly well.

There's also the sheer matter of scale as well. In a world where millions of people are required to operate a single warship, there's also the general issue of finding a way to balance an interesting character that you want to use to push a narrative forward, with the fact that realistically speaking, even the most 'interesting' or 'senior' characters people play are the tiniest cogs in the machine.