I have to admit that I have not played more than a short time with a friends 5x so what I know about that phone is mostly anecdotal. 3 friends bought the device all 3 have moved on. Read
Nexus 5X, three months on | Android Central of 5 reviewers, 5 site lag and poor performance, only 1 says not to a point where it's a concern.
I believe that the SD 808 and 810 will be looked at by history as a step back for Qualcomm. It was more than interesting that Samsung reportedly stopped development on the 810 after testing found the processor wanting, and simply not as good as it's in house Exynos, but that they went back to the 820 with the S7. I own both the S7E and the OP 3 and both phones sail - the 820 is more than just a step up in performance it's a much better package and lends to a much better user experience just on it's own.
I also have to disagree some with your blanket statement that the Nexus QC is better than Dash. Dash is extremely fast and what it does do is take the management off the phone. I did not have a Nexus as I said but I did have the BlackBerry Priv running QC 2.0. Plugged into a QC 2.0 certified charger the phone would often get extremely hot especially if it was in the car running GMaps and PocketCasts at the same time, while charging The phone would obviously throttle at times and get to the point where it would just lock up, especially if a call came in in the midst of it. Removing that thermal challenge is going to be a benefit to the phone without a doubt. We will probably agree to disagree on this one.
The N5x wins audio because it has a front facing speaker that many describe as "tinny" (see the review captioned above). The bottom facing speaker on the OP3 is actually fairly good for a bottom facing speaker. Let me quickly add as well to watch the pocket now review and their comments about the headphone jack on the OP3.
Finally I know you think this is subjective, but eventually dark mode will find it's way back into stock Android. The OP3 has it in it's software. I also like the notifications toggle. Also subjective.
As I said earlier I think the OP3 is a package that works better than the sum of it's parts, I feel (again based on anecdotal evidence) that the N5x might be a package that does not live up to the sum of it's parts. I know they've made it better since launch, but I still believe there are persistent problems that nag that phone.