The adventure from the GM kit is best suited for mid-level PCs- it is not
an introductory scenario.
While the adversaries are fairly unpleasant, I don't think this meant that it was intended for mid-level PCs. Black Industries seemed happy enough throwing PCs into unfair situations that if they tried to take on in a straight up fight they would just die. I know of one group who dealt with the problem by going "Erm... there is something hideous in that windmill... lets set it on fire."
Even if you didn't host the Haarlock Trilogy, House of Dust and Ash (back of Disciples) is pretty solid. Definitely not for greenhorns, though.
I have never had a chance to play it, but I was going to use it in my campaign if it had lasted long enough. I like the overall feel, but yes, not for beginners. Firstly, the approach towards the PCs does seem to be that they are more trusted servants at this point, given a task that needs some discretion, rather than being thrown in because there is no one else, or just to see what happens (ala original the free RPG day adventure, which certainly feels like the Inquisition could care less what happens to the players).
Since we've already derailed...
Making DH Space Cthulhu was a bad design choice in the first place.
The epicness that is 40k is completely washed out it's not even funny. What would have been fun was a combination of over-the-top action, cosmic horror, and investigation. Pretty much what later products try to deliver.
I wonder who first came up with the brilliant idea: Hey, lets make an Inquisition-based RPG, only we'll make sure the PCs are really, really crap.
I've had players comment on this, nothing that it's no wonder that the Imperium is failing if these acolytes are all that stands between Humanity and Utter Ruination
I far far prefer the feel of the original Dark Heresy stuff. I want to play in the dark and grimy world of 40k, not play Space Marines etc. I want to feel the world is a horrendous place that is out to get you, and you are just struggling to survive. Now, this can change (and higher rank Dark Heresy PCs can have quite potent abilities, even without the silliness of Ascension), but it should be goddam earned.
The worst of the series has to be Deathwatch, and this is the one that tried to get the whole epicness the most. NOw, there were various reasons for this. 1) The game breaks down at this level (the inherent limits of the mechanics become painfully obvious), 2) FFG made some boo boos with their choices re the mechanics (dull dull horde rules, silly weapon stats, incredibly difficult to understand squad modes etc) and 3) Space Marines make terrible characters to play, and the adventures they will tend to go on get very repetitive, no matter what FFG tried to say. Space Marines are super soldiers, and their primary mission is to go out and murder aliens in whatever way possible... this gets slow quite quickly. A bit of variety can be added, but it only goes so far, and doesn't reach the variety that DH and RT characters can have. It is also the most dependent on setting knowledge. DW characters should have a handle on how they should be played, and on what is going on. DH characters shouldn't know what is going on (at least at the beginning), so a lack of player knowledge is actually in keeping with the setting.