the scaling issues. Qtum isn't anywhere near capacity yet. It has lots and lots of room to grow before network hits full capacity.
The UTXO transaction model that Qtum shares with Bitcoin, is the most proven and secure model for transactions. It also allows for lightweight clients and it's why Qtum has first class mobile wallet support.
Qtum is truly decentralized unlike NEO as someone else mentioned. It has 2000+ nodes on the network. That's impressive for a mainnet that is less than six months old.
As for scaling solutions, Qtum can implement Lightning or it can implement sharding as Ethereum is doing. Or a combination of both. Lightning is more for payment processing. Sharding would be a solution for smart contract execution.
As stated above, Qtum already has 3x throughput of ETH network. That's impressive on it's own. Long before Qtum hits the limits of it's network, we'll see these scaling solutions fleshed out.