This page shows how many machines people are using on there, per algo:
https://www.nicehash.com/algorithmSo it looks like conservatively there's 500k machines running on there, and even each person had 10+ machines that means there's tens of thousands of people using it again.
Is it safe? Probably more than some pools out there... Could it get hacked again? Sure, but so can someplace that hasn't been hacked.
If you like what they offer, and their fees are acceptable for you, then you should use them.