I'm on my second playthrough now, this time as Durge (right up to the hard decision you mentioned in Act 2, so can't say anything about Act 3, but because of my Tav playthrough I can imagine where the story is heading).
I agree mostly with you, after the scene in Act 1, there is basically nothing much at all happening for Durge. My character wants to resist the urges, but even so, it feels a bit too easy to do so (just click that option). Theres should be more of a tradeoff there (or at least some checks).
Also, some of the Durge story/dialogue is outright funny/weird, like when you run up to all your party members and tell them basically "I'm a psychopath who needs to slaughter people, do you mind?", or "I didn't do this" after they clearly saw you doing it (in my case, failed the test to hide it). They might have to smooth out those a bit.
On the other hand, I think it's ok if you miss out on some of the stuff (like the quest you mentioned) as Durge because you do get another powerful item right at the start, and playing Tav should also have it's perks. It's not bad in my mind that you can't do everything as either Tav or Durge, but that there is a bit of a tradeoff. If you play as a mostly resting Durge you don't lose out on much (so far at least, the only thing is the one you mentioned, and I got something else in return for that, which I also can't get as Tav. Feels fair to me).
A real evil playthrough is not worth it yet I think. I'm doing that with a friend, and in that case you lose much more than you gain (even if you don't play as a murderhobo). Like siding with the Goblins is not in any shape or form equal to siding with the druids/tieflings (quests, experience, items etc.). But that's true for both Tav and Durge.
