I have been a Merlin user for quite a while. The AC86U is my third Asus router and I manage a couple of AC68U's in a small not-for-profit office. I have had wonderful success, for the most part, with the Merlin firmware. Sure, there have been bumps along the road which have been ironed out in time. But since I got the AC86U earlier this year I have had several times on Merlin where the internet connection just stopped working overnight. Since going back to the Asus stock the router has been rock solid for me. I do miss the Merlin features but feel I need to weigh reliability and security. I have added a Pi-Hole with Stubby DoT to do DNS filtering and that works very well with the router DNS as a fall back. For now, it works.
The one issue you will likely have switching to Merlin is the /jffs not mounting. I have read that, with patience, you can overcome this and get your AC86U to save settings. Read through the various forum posts about this.
Try it if you must but get the Asus router restore software and a copy of the Asus firmware just in case.
Best of luck!