You can not spam stuff like he does, and can not pull long CD abilities spammed.
Neither can a Tempest Rogue. AS Archer you get, what, 3 Long shots off? As Dual wield you get 4 abilities off? Then you wait for quite a bit. As Artificer you don't wait. it might take a second longer per longshot, but you keep doing it over and over again.
rit bonus/Damage bonus of Arty are neglected just by nightmare chaining and having near to constant crit on your rogues.
High crit benefits artificer more than anybody else. Assassin is the only spec that might "lose" on high crit. I speak of the passive that guarantee crit out of stealth. if anything, this point speak for Artificer, not against it.
Tempest Can: Get in and get out at whim.
Which I agree on. A tempest rogue is the most jack-of-all, and gives you more control of the battlefield. However, killing an enemy is the best escape you get 
My point in all of this: Artificer is gold. It's not about traps. It's not boring. It's about having near no CD and gain stamina back at an insane rate. It's about having a consistent longshot spam (the time it takes to fire is enough to reset the CD). That's 4k-11k hits every 2-3 seconds sustained. Basically:
Assassination: Mark of Death and armor penetration is the only thing I find interesting here. Both are amazing though! A very good specialization, and the highest burst potential. Enforces the stealth - burst - playstyle of rogues.
Artificer: Highest sustained DPS. With enough crit you are annihilating the battlefield. It does not change or enforce playstyle. It just is there, making you awesome.
Tempest: Flask of fire? Artificer passive is quite a lot better. Flask of lightning? Good for escaping, but not a true dps increase if you control the character. Flask of Frost? Fantastic for survivability!... which is not a problem with a competent team setup. It changes the playstyle from "stealthy" to "I do a bit of everything!", which is nice. Very good for those who do not likes rogues stealty-play.
I think all of them are good. However, I do not think tempest beat Artificer in a pure dps match. In fact, I think artificer has the possibility to do the most damage of them all, excluding focus abilities. I also think Artificer is the most underrated and misunderstood specialization in the game. At first glance it seem trap-focused, but then you look at the passives.. traps got nothing to do with it. The synergy between crit, stamina regen and CD reduction is insane.
In terms of efficiency, I rank Assassination as best for Dual Wield and Artificer for Archer. Tempest is the special snowflake here, for those who want to change the playstyle, instead of enforcing the one you got without a specialization.
Do note, however, that this is all min/max discussions. The fact we can kill a nightmare dragon in under 30 second is a sign it's all rather irrelevant. Fun, but irrelevant.
So. Yeah. Tempest is not godlike. Artificer is not bad. Assassination is brokenly good. Rogue is the master class.