Some people have argued that exchanges could be delayed by sufficient confirmations, but remember that with proof-of-work a lie-in-wait 51% attacker can rewrite the entire block chain.
I think that any kind of attempted solution of any problem is subject to the same "criticism".
"Sure, this works... but remember that 51% attacker can rewrite the entire chain"
The distinction for Side Chains is one block chain is subject to the 51% attack of
block chain.