The sharding solution they have proposed, whereby contracts can only interact in the same shard, will work just fine.  That of course brings with it usability issues, whereby a contract/account on shard 5 can't interact in any way with a contract/account on shard 10.

Enabling contracts to interact across shards is a different level of hard altogether, and I'm at the tipping point of accepting that it can't be achieved after looking at the problem myself for the past year or so.

As many know, eMunie has partitioning (our name for sharding) of transactions, and these partitions can interact with accounts in a different partition.   Great you might think, then just apply the same mechanics to contracts.  

Unfortunately its not that simple, as with transactions you are managing a single, known state (the balance) that is native to the system, so you are able to build functionality and algorithms that leverage those properties to enable cross partition/sharding interactions.  With a contract its much more complex, as a contract may have many states, all of which will have an unknown/indeterminable function, and be arbitrary in nature.  

If the states of a contract are arbitrary, the system can never know what all the states might be (because they are defined by the contract creator).  Nor can it know the rules that govern them due to the same reason (the rules too are arbitrary), thus there is no way to enforce consensus across shards.  Therein lies the root of the problem.

For this to be possible at all would require that the system knows, natively, all possible states for any contract that may ever exist, and the rules that govern those states....