In my opinion, Adonalsium as an individual is an already a solid plotline. The different Shards represent the many facets or personalities an individual has. I like to think of it as an adult version of Inside Out except the emotions in the protagonist's mind Shattered then became different Shards thus forming their own identities? Also, as you've mentioned, the powers fused well together - initially. However, Sazed also mentioned in Era 2 how he has great diffiuclty acting as one Shard with the powers of Preservation and Ruin as they oppose one another. This can also imply that they can fuse really well at first but they also need the other Shards to function well together - as kind of a neutralizer.