I used to use dd-wrt, but I later settled on ASUS routers, and installing "ASUS Merlin" firmware on them.

I find ASUS hardware high quality in general. And using an open-source project like ASUS Merlin means the pain-points and limitations that I would face with stock firmware have been resolved to (at least to satisfactory extent) when I need reach for the hacker type configurations.

How does DD-WRT compare to RouterOS?

IMHO, those who want a router with open-source firmware are going to have the technical knowledge to do the finding and flashing themselves, by virtue of being embedded in tech deeply enough to recognize the value of an open-source firmware in the first place.

Those who look at you blankly when you mention open-source router firmware aren’t likely going to become customers.