Okay so having played Dual Wield Rogue for about 40 hours now on Nightmare, and at about level 15, I can say the following:
Melee Rogue is very tough if you don't have a specific plan. If you're just bumming around looking for flank damage you're not going to do well. You need a specific strategy to avoid damage and to deal damage.
The setup I've got now is as follows:
- Tempest specialisation
- Pretty good daggers, nothing amazing. One made of Stormheart, the other of Serault Glass, both with Critical Chance improving grips (my crit chance is at 40%)
- Knives skill from the Poison/Traps tree.
The plan here is to activate Flask of Fire as soon as combat begins. This allows you to spam Knives 4-5 times, doing fairly massive damage on 2-3 enemies and sundering them. The rest of your party (I have a 2H Warrior and a Sword and Board tank) charges in while you're spamming your knives and draws aggro. When your Flask runs out you just start circling the edge of the melee, looking for a flank to start attacking, preferably with spinning blades. If you have Ice Flask use that as well to pretty much keep you safe. Once Flask of Fire cools down you just do it again.
Very simple plan, doesn't require optimised builds or equipment beyond just common sense, and requires a decent amount of situational awareness, but nothing insane.
I'm currently having no trouble at all with same-level popcorn fights, I'm able to deal with all challenging/boss fights with proactive use of potions and tonics, and am regularly mopping up over-leveled (level 17) random encounters too.
The reason Dual Wield Rogue seems more difficult is that you need to have a plan for dealing with enemy attacks before you can succeed, whereas an archer just stays out of combat entirely. DW Rogue needs to either use Flask of Ice or Stealth or Evasion to avoid attacks, which requires more forethought and active engagement in fights. I tried Archer rogue and it was pretty boring, honestly. It's still okay, because you still have to choose targets intelligently, try to get to higher ground, or keep distance to get your Long Shot bonus active, but you just feel so removed from the action.
Dual Wield Rogue is absolutely viable, and actually quite effective, as long as you have a strategy. There are others that work besides the above one. I'm pretty sure an Assassin with Hidden Blades (a ridiculously powerful skill) would also be effective.