Scalibility should not be a problem for Qtum. Although contracts can create state changes etc, these state changes are not directly stored on the blockchain. Rather, state is considered the accumulation of all actions and differences created by executing each smart contract. This could admittingly make syncing the blockchain slower (because block processing will be slower than Bitcoin), but the other benefits provided are worth this performance hit. Furthermore, the only additional thing stored in the UTXO set right now is contract code, and contract-owned outputs (though their data becomes unimportant after the output is initially processed). There may be plans proposed later which prune the UTXO set’s stored data as well, to reduce the UTXO set size further.
Where is the contract state stored if its not on the blokchain