Dark Heresy combat is definitely extremely clunky, but part of this is probably just your players. In my experience it does indeed take two or three months of consistent weekly play before people get any kind of system mastery, and that is atrocious, but it does actually happen. There are people who can't grasp the idea of adding their MAB to a melee attack after six months of playing D&D, and the problem with that is not that "to-hit rolls are d20+MAB" is extremely complex, it's that the player isn't putting forth any effort to learn the system.2 years of playing almost weekly and my players never got the hang of combat. It was always clunky as hell.
I'm okay with Dark Heresy following the trajectory that you start as a mostly random schmuck and level up to become a hardened Inquisitorial agent. DH1 did not deliver on that premise, but DH2 did so as far as is possible within the constraints of the d100 system. The rules for becoming an Inquisitor are core in DH2.Your characters are just random schmucks recruited to serve as expendable pawns or deal with low-priority shit, and whatever the Inquisitor who pressed you into service does, he's probably nowhere near your party. You can easily be gunned down by random thugs. To play an actual Inquisitor, and generally be a badass (without using psyker powers to break the system), you need to use a special supplement that adds enough extra complexity to the game to essentially make it uplayable.
I really intensely despise the lack of consistency between different game lines. It requires way too much conversion for a Deathwatch space marine to go rogue and become a Black Crusade character, or for an Imperial Guardsman in an Only War campaign to migrate to Dark Heresy, where Imperial Guardsmen are also supported characters (Only War to DH2 transitions really well, and they both transition moderately well to Black Crusade, which is nice). Plus, while Black Crusade does have a lot of sneaky cult stuff that it gets up to, it is in fact named after massive invasions of entire sectors (or more) of Imperial space, so that splat in particular very much needs to do large combats well despite the vast majority of its combats being a party of 3-6 versus like eight dudes.I think the game could have benefited from making different rules for combat for the games that focused on larger combat (Only War, Deathwatch), and kept the more complex rules for the more combat-lite games (Black Crusade, Rogue Trader, Dark Heresy).
Having it be a 0-100 characteristic like the others is a good idea for consistency's sake (although having the others be on that scale is a bad idea for reasons discussed above, but in fairness to FFG Rogue Trader was their last chance to walk away from the d100 roll-under system without serious damage to their brand). Having you actually roll that characteristic to get stuff is easily the thing I have most frequently seen complained about. Really, this is just an extension of the fact that d100 is a really swingy and unpredictable resolution mechanic, but people actually seem to immediately notice that when it affects their ability to buy things. I'd have Infamy/Influence/Profit Factor/whatever (we're just going to call it INF from now on since that works for two of the set) be a set of ten levels from 0 to 9, determined by your INF bonus. So if your INF is 44, your INF bonus is 4, and that's what we care about. It means you can automatically buy anything of with a -30 availability penalty, but -40 or more you have to roll for, although you'd have to calculate the availability penalty differently, probably doing it from your bonus rather than absolute. So the guy with a 44 INF who wants to roll for a -40 item actually takes a -0, because his INF bonus equals the item's penalty bonus, and for a -50 item he would take a -10. Then you would need to redo the availability on every single item, because items with -30 penalties have just gone from being really hard to get your hands on to being automatically available to someone with INF of 40 or higher.Any thoughts on how the Influence/PF/Infamy system was implemented?
Although this is all a bit more complex, I think it's worth it to give people, especially high INF people, a list of items that they can just have without any rolls at all. It's like, yeah, I'm super rich, you want some carapace armor, I don't even have to roll. Power armor? Well, I'll have to roll, but my odds are like 60% so we should be good. That's a lot more satisfying than having only 70% odds of getting carapace armor even after you've maxed out INF almost completely. Which, again, kind of highlights how a d100 system doesn't mesh well with the 40k setting at all, because a Callidus assassin's odds of sneaking past a Guardsman isn't 80%, it's "yes."