It is assumed that the shards on each planet gave up some of their power to create the life forms on those planets. It may be that Odium did not aid in the creation of life on roshar though, hence not giving up any of his power.
Not only that, but all Shards are not created equal.
He holds the most frightening and terrible of all the Shards. Ponder on that for a time, you old reptile, and tell me if your insistence on nonintervention holds firm. Because I assure you, Rayse will not be similarly inhibited.
Granted, the author of the letters *Cough*Hoid*Cough* could just be commenting on the nature of Rayse's shard (which makes sense if he holds Odium). But it does sound like his shard is a little more epically awesome than the others in terms of power.
Also, just because Ruin and Preservation were equally matched, or close enough that one couldn't kill the other without being killed, doesn't mean that we should assume that all shards are this way.
And lastly, though it has virtually no basis here, since this is all pretty metaphysical anyway, in my experience, when you shatter something, it's pieces aren't the same size. Even assuming that Adonalsium shattered based off of personality traits/characteristics as seems to be the case, certain traits would be more dominant in Adonalsium than others. Although if this is the way it works, it's telling that Odium is one of the more powerful Shards, and that's on top of the other messed up ones like Ruin.