Some character creation combinations seem a lot better for the "general" situations like gathering information, barter, etc.
Our group has a Highborn Outcast Assassin who was already equipped with most of the skills needed for social interaction, for spotting hidden stuff and so on, because of starting skills and the right combination of aptitudes.
So when he was gathering information from the locals the rest of the group was pretty much standing by.
Same for the infiltration when sneaking was required. The Adeptus Mechanicus opens the lock but only the Assassin sneaks in, the rest of the group (Psyker, Soldier, Mechanicus) waits outside until they felt it's safe to follow, as no one seems to be home. We were wrong and of course screwed up the first unskilled Stealth test, the shooting starts.
So what am I saying?
The Tech aptitude is nearly useless, as it's used only in 2 skills and 3 talents (of which only 2 seem worth the trouble imo), unless you play Adeptus Mechanicus. That means the Seeker role has one aptitude which doesn't help a lot at all unless you want to concentrate on being the Tech of the group and mainly raise Tech Use. Yay.
Similar is Strength with 2 skills and 5 talents.
Compared to Intelligence with 11 skills and 12 talents (11 if not counting the special Mechanicus talent) or Weapon Skill with 1 skill and 18 talents (17 if not counting Mechanicus special).
That means that our Highborn Outcast Assassin starts with many useful skills from his Outcast background and doesn't have many problems advancing them either. It's more expensive for him to get the Tech Use or Security skill, yes, but advancing social skills and fighting skills is no problem, so there's still enough XP for the "knowledge skills".
For our Feral World Imperial Guard Soldier it will be very expensive to get most besides fighting skills and playing a social or fighting-oriented Mechanicus will be expensive too, so they'll either stay very specialized or advance much more slowly than the Assassin.
Having most aptitudes come from the role only means that we have characters who aren't very good at what they used to be doing before they became Acolytes (because starting talents and skills aren't that much) and can most easily advance in the direction they are assigned by their role some time ago when they became Acolytes, almost no matter their former training.
Having more aptitudes come from Background would make more sense to me. More customization could be included here, for example let the Ex-Imperial Guardsman choose his former specialty (see Only War) - the former Seargent gets Fieldcraft + Fellowship, the former Medic gets Fieldcraft + Intelligence, the former Heavy Gunner gets Fieldcraft + Toughness.
Just as the Ex-Adeptus Arbites gets Offence or Defence, plus 1 aptitude related to their former specialty (Fellowship for the Chief, Strength for the Enforcer with the hammer, Perception for the forensic folk and so on)
Long story short:
Dividing up the aptitudes between Background and Role (and giving some chance to customize) would probably make for some more balanced characters and would make character creation choices more easy too, when having later progression in mind.
Background for this post:
I ended up creating a Feral World / Imperial Guard / Hierophant because I wanted to continue the story of my former Only War character (-> Feral World / Imperial Guard) and be able to actually help in other situations than fighting. First idea was going for Seeker (could have been a bounty hunter after the Guard) but the aptitudes would have me stuck with the little fighting skills I had from the beginning for a while. Looking at the aptitudes Hierophant gave me a good balance between fighting and social, so that's what I ended up with. Granted, I'm happy with it and I have an interesting character story.
Sorry for the text wall, folks