I'll add one point here, which is that the Scum's generalist nature REALLY makes them a power play character in smaller parties. In my first campaign, our party consisted of only three characters. For those saying: "well just get other characters to cover different niche's", in a two or three person party, it's doubtful you'll be able to cover the same breadth of niche's as the Scum does overall. Additionally, early game, the Scum gets some very good early game perception (Awareness+10 at rank 1 and Deceive+10 at rank 2), that few enough careers can actually keep up with. Plus in the early game, the Scum can start with an autogun which my group continuously struggles to balance encounters around, especially once the Scum gets access to custom ammo like Manstopper rounds and feats like Mighty Shot.
Are they outdone in SPECIFIC catagories? Probably, so there's probably little need for a Scum in a party of 5+ players, and they'd probably be the fallback guy in a party of 4.
But in parties of 2-3 players, they're pretty absurd due to their flexibility.
Last DH game we ran was group funds, looting bodies, and attempting to muscle in and "steal" things ("You won't mind if the Inquisition borrows this, will you?" it helepd to have a guy in the group with a lot of charm, and another one with quite a bit of intimidate).
Yeah, I've been kind of dissapointed that a huge chunk of my DH campaign revolves around my players looting bodies and trying to steal and sell stuff. The next campaign I run, I think I'll start out with the Assention rules for aquiring equipment right from the get-go. Obsessing over coin is for D&D...
I think the answer would depend on what type of world you've put your players into. If there's a bolt pistol available on every street corner and the acolytes have blank cheques signed by their Inquisitor, not being able to pick up bolt weapon training at level 1 seems like a waste. However, in our group's current game, the assassin who trained in Las weaponry is now level 3, and is still unable to find a Long Las rifle (due to a combination of poor rolls and the scarcity of the item).
It also depends on the type of missions the players are thrown into. Placing them into an investigation-oriented mission on a hive world may give the scum a lot more use out of XP spent in Common Lore (Underworld) and Peer (Underworld) than a guardsman's melta/plasma weapon training with the dual shot talent. Even an adept, with their knowledge of the administratum, imperium, cults and legends may only be able to help so much if the main problem is an underhive ganger boss sowing the seeds of insurrection.
It's worth pointing out that the scum isn't a 'rogue' from classic RPGs: it's not an assassin or thief necessarily. They can focus on becoming an information gatherer or a quick-drawing pistol wielding combat character, and can excel in either roll. Like pretty much every class, they will feel most valuable if they're filling out a role that the party is lacking. At 2500XP, mid-4th level, a combat-oriented scum could choose +10 BS, +10 AGI, and +5 T, 5 extra wounds, hard target, hardy, dodge+10, awareness +10, silent move, concealment, security, and quick draw. Gear could be an Amegeddon autogun from the Inquisitor's Handbook with manstopper bullets for 1d10+5 damage and 3 penetration. Fraction of the cost of a bolter for nearly identical firepower (arguably more, as the autogun has a full-auto option). I wouldn't worry too much about this character falling behind.