Then that raises a question of why play a Inqustion roleplay instead of only rp that can be of only one faction subgroup such as only a single imperial guard regiment or Astarte's company or even sorritas order to make sure all players are intertwined and will work together.
There are inquisition groups of mixed individuals which still can work together without slandering each other all the time. Ordo Xenos under Veronica went pretty smooth in my opinion. Despite 2 aliens (even 1 being a secret alien)
Well, considering its the inquisiton, paranoia is also the thought of the day as they must look for the subtlest hints of heresy and corruption, which isn't as obvious as one expects and can easily come from unsuspecting sources. And there's plenty of times where said branches had attacked one another due to claims of heresy both drummed up and real. There be harder common ground to find with those of completely different backgrounds that would be normal for one character, but practically heresy for another, like how a guardsmen might operate compared to a sister of battle. Where a guardsmen would find it prudent to withdrawl and reinspect their plan of action, most sisters would just continue to attack till death of either them or their enemies, finding it Cowardice to do anything but. I think that might be what he was referring too.
Well since I'm an investigator, my character is both a lone wolf and a team player so he's a wild card to begin with since Investigators normally are required to infiltrate without backup but they're also still a frontline officer so they have the skill sets to work as a team. I would find the Astartes to be the ones most likely to jeopardize a team mission since they have a history of not working well with non-Astartes.