Qtum's flaw is similar to many full decentralized blockchain like bitcoin, ethereum. it suffers from scaling issue. Qtum right now can do around 70 tps which is about 2 times as ETH can. this number is quite impressive but still not enough... however there're many scaling solutions in the work such as Lightning Network. moreover Qtum right now can scale with DGP by voting to increase blocksize and decrease blocktime but DGP also has a limit, it's not practical to increase blocksize to something like 1 GB to support more TPS, right ? since Qtum is technically a bitcoin fork so it benefits nothing from any innovations from ETH camp. for example if ETH team releases some cool things like Plasma, Qtum will get nothing from it because it uses a differen model (i might be wrong about this, anyone feels free to correct me), this might be a problem because when it comes to how to scale blockchain, i give most of my trust to ETH team. they show their strong intentions to scale blockchains unlike bitcoin core team who just changed bitcoin's slogan from "digital cash" to "digital gold".