The real problem of a 10 month roi is that no one knows what will happen with the prices of btc, eth, xmr or any other coin, maybe in 3 months the prices start to rally again and the roi goes from 10 month to 3 months, or maybe the market continues to selloff cryptocoins and the roi goes to no roi because our rigs does not produce any profit.

So when calculating roi seems people is thinking the current prices are going to be stable, and I think this won`t happen, so is not possible to know the real roi of an investment in a gpu mining rig.