Not entirely true. I have an alchemist in my party that sits in the back and doesn't attack often (pellets are limited, I find) and he gains experience in every battle regardless, to the point where he's equal level and experience-point-progression.
Oh, I guess my alchemist's trailing behind might be entirely attributed to the experience bonus of my humans then. Sorry for the misinformation.
What I have noticed is that there seems to be far less randomness (if any at all) in character advancement. Health and energy increases seem to come in a steady amount instead of being rolled. At least when I reloaded a save game before leveling to see what happened to my hp, they increased by the exact same amount. It might be because my characters' vitality just isn't high enough to make a difference yet, but it looks like a positive change has happened in my eyes.
It no longer pays to invest in skills and attributes in a very specific order to get better stats anyway (athletics doesn't increase vitality anymore, it instead directly affects your HP), so there's less room for min-maxing even if some random factor still exists.
Off-topic: is it just me or are firearms quite weak? The sling does more damage for me because it gains a bonus from dexterity. The pistol I found doesn't get a bonus from any attribute. So it would only really be helpful for low dex characters who still want to use a ranged weapon that isn't thrown.