Only bad thing that could happen is if someone deploys a few PH/S and difficulty rises out of control. It's not likely, but more people looking at btc =/= more people mining.
That what we probably will experience soon - after HashNest put on the market S9.
More than 1000 unit are already sold.
Before HN controlled difficulty buy mining in full capacity first 9 days after diff change, and slowing down last 5 days. We had moderate ~3% increase.
Now having on miners hands tens of PHs the control is little bit more tricky.
It seems the difficulty will rise 3% or more every 14 days.