You might be right, maybe I played it wrong. I used the build of Penguin and he seemed to think as I thought too. Could you share your own build? Would appreciate your suggestions.
For the daggers, both are above 20 (a rare and a unique), and my DPS was always very decent, so they're not the problem.
The problem was that any small mistake (or the DC showing up, or the Red Templar boss earthquake hit when I was killing an archer) I was dead. Even during the momentary switch from flank attack to invisibility, I could find myself with 10% of health vs beginning of the move. I could show up in the 1st of 2nd place if the team was good (because I was doing lots of clean up), but I never felt like I could solo even in Threatening difficulty. This class seemed always like a suboptimal assassin to me...
Dont get me wrong, I know how tricky it is and the smallest mistake can really cost you. I also tried out Penguin's build and it kicks butt. I think this character, as with the Assasin, it is about placement and timing. When it comes to RTC and DC, here you need situational awareness. I do not mess with them. I see my job as focusing on the adds, while keeping tabs on the boss, and players, should they need a revive. With RTC do your best to keep an eye on his ground attacks, using movement and jumps to avoid (same with DC).
When I am playing I stealth, run to back of enemy group, and begin my attacks, taking full advantage of the 0 threat flank passive. Sometimes you don't get the kill and so you have the option to run for cover, or try for a quick kill that will bring stealth out of cool down.
For accessories I tend to use: amulet of cunning, ring of life drain, attack ring, +health belt. They help, but if you don't have them you can still do well.
A couple of build tweaks you may want to try as well.
Stealth, twin fangs/death blow, flank attack, knock out bomb
Stealth, fire flask, death blow, knock out bomb
To recap, positioning and timing are key. Risk management: which enemy can I take out with the least amount of risk.
Granted, no matter which character you play, the team composition does make a difference. If an archer for instance is long shotting everything, you may be hard pressed to get the kills to bring stealth of cool down.
Practice that: risk management, position, timing. To me they make all the difference.
Keep at it. With enough practice, mistakes and successes, you'll. Be rocking the Alchemist in no time. Once again said by someone who once upon a time thought her a weak character with cool lines.
All the best.