IIRC, Chromatic Harness doesn't improve the CD of one breath : the unlockable one. Venomous breath right?
Venomous Breath is more or less the best breath even without the CDR, the % chance to fail actions is an incredibly strong effect. The main problem with it is that the entire breath can be resisted, unlike the others.
That's not a little problem. It also can be cleared by a wild infusion. Most humanoid enemies start with random inscriptions so even having unbeatable mindpower doesn't make it stick reliably. If 5% of thieves have a physical wild (they only seem to get infusions rather than runes) and everything is a rare that's a lot of nasty enemies that the breath falls flat against. It's better than when it was insidious poison since it actually accomplishes something when it works, but it's probably still the worst breath.
Fire is probably now also completely resistable unless flameshock was fixed without showing up in the release notes since it now gets a chance to flameshock and flameshock fails against stun immune enemies.
If I were going to pick two breaths to skip those would be my choices in a vacuum, though poison is on the way to key multihued talents while acid, fire, and to a lesser degree lightning are in completely skippable trees unlike sand and ice with utility powers in the third slot that are well worth their place on the talent bar even at one point.