Lumia 950XL has serious issues with 5GHz ac wifi. It causes some devices (like mine) to randomly reboot. I also noticed disconnects from wifi. If, for some reason, your wifi loses internet connectivity Lumia will automatically switch to LTE/4G and frequently will not come back to wifi, until restart.
I noticed that Windows 10 in general has a lot of problems with wifi. All of my devices (HP laptop, HP tablet, Surface Pro2) have all sorts of issues with wifi. The worst being the tablet - after going to sleep it will not recognize the wireless adapter at all. So, I think this problem might be more general than just our Lumias.