Right now the ROI times are so short... that its a very low risk at the moment buying rigs.
But I agree with you. Problem is nobody knows when the ETH bubble will burst from all those scam ICOs.
I don't agree with "very low risk" A ROI from here is certainly possible but I think the risk is now very high. Prices of tradeable assets usually decline much faster than they rise and a sawtooth pattern is often evident. The massive increase in coin prices in the past couple of months might evaporate in a couple of weeks. It's entirely possible prices will be 50% lower than now in a few weeks time. But there will be no corresponding decrease in difficulty. What was very profitable 2 months ago, will be much less profitable at the same price now.
ROI based on current figures is around 2 to 4 months. The problem is that current figures are pretty meaningless on that time scale. Coin value needs to rise continuously to get your money back in that timeframe because difficulty definitely will continue to increase. In a falling market or even a static one, ROI will be much more than a few months and may never be hit.
Prices may go up and down but difficulty will rise inexorably. The long term (measured in months) profitability will likely become an annual yield of around 12% of asset value after paying 10 cents a kw. Those with higher electric will lose money.
The coming bloodbath will be interesting.