I do think Harmony is much more dangerous than he appears, at the current stage. A lot of the Metallic Arts works on a Newton's Third Law 'action-reaction' level, and the real issue Harmony has is that whenever he wants to use Preservation without Ruin, Ruin counters it, and vice versa. But if he can manage to use both at once (therefore maintaining the balance), say by defending against an attack, he can unleash his full power.
Having said that, I believe that Sazed, as Harmony's Vessel, will find it harder and harder to act independently, as is the nature of all Shardic Intent over time.
I'm very curious as to the polarization of the Cosmere into warring camps. There definitely appears to be at least three sides to the conflict: Odium and those who follow him, wanting to destroy (or perhaps at this point absorb) other Shards, Harmony and those who follow him defending against Odium's side, and the third neutral side, who I feel are being led by Endowment (among the Shards) and Frost (among non-Shards). I find Endowment's Stance baffling in the extreme.
Anyway, regarding the 'sides' of the Shards:
Odium's Side:
Odium (naturally)
Autonomy (pretty safe bet)
Mercy
Harmony's Side:
Harmony
Valor (possibly)
Whimsy
Neutral:
Endowment
Invention (I believe Invention is in Silverlight)
Wild Card(s):
Cultivation (seriously wtf)