I can't speak for the z170 board, but my z97 extreme 4 has been very sold. giving me a winter 4.9 ghz profile on a 4790k. I can even get 5.0 stable but the volts are a bit much for me to be comfortable.
I used to think of asrock as a poor mans ASUS board, But i believe they're getting quite good fast. Not as flashy, without all the bling, but a better bang for the buck IMO.