We don't have hard numbers. But based on the groundswell of negative reports on this forum, it certainly seems like the S9 has had more reliability problems than previous models.

I only bought a couple of S9s early on. One has a bad hashboard.

It's the first Bitmain hardware failure I've had since I started mining as a hobby. I had a couple of S3s, a Spondoolies SP20, and three S7s without a single failure.

I realize that my numbers are not statistically relevant. But from a personal and anecdotal perspective, the S9 has been less reliable.

That said, I still got to ROI with my S9s. Barely though. The board that crapped out happened about two weeks after that unit ROI'd.