In real life, I agree. There does, however, seem to be some sort of genetic...predisposition. All the Legions (to my memory) were roughly in line with their geneseed from the get-go. Generally meeting up with the Primarch merely reinforced and refined their habits and skills.
This makes me believe that the Primarchs were genetically predisposed to certain traits.
Agree. Perhaps the Emperor could of made those predispositions less pronounced.
We also would perhaps seen the lost 2.