The problem with Chantix (varenicline) as I see it is that it isn't enough of an antidepressant. It doesn't substitute for the energizing or mood-balancing effects of tobacco/nicotine.
Varenicline is a nicotinic partial agonist with no clear action (as far as my amateur reading can tell) on the major neurotransmitters implicated in depression, such as serotonin, norepinephrine, or dopamine.
Chantix will reduce the effectiveness of smoking on the nicotinic receptors by competitively binding to them, without allowing the release of dopamine that nicotine usually creates.
It just shuts down the pleasure response without substituting for the antidepressant effect many people get with smoking.
(I don't know if it would reduce the action of the other alkaloids in tobacco. But people who find Chantix "merely" ineffective might be a group that smokes for more than the nicotine...? Of course, those who find it intolerable may also be part of that group.)
The upshot is:
If you've been using smoking as an antidepressant (consciously or unconsciously) for years, and you therefore don't show up as clinically depressed by the HAM-D test or a doctor's diagnosis, you'll be given Chantix because you're "not depressed".
As soon as you quit smoking, you'll give up your antidepressant. And Chantix will block off the nicotine's ability to help you.
No wonder it's dangerous.
(Wellbutrin, on the other hand, is an antidepressant, but it's a rather unbalanced one on its own, acting mainly on norepinephrine --> for some people, anxiety land. It's also a nicotinic antagonist. I took Wellbutrin for years as part of an antidepressant combo. Got a late start smoking...when the Wellby crapped out on me! And it got a lot worse when I stopped the Wellbutrin altogether.)