Vampiry; Long ago, in ages past, 'BFE' (Before the Merlin Era) we owned a nice-looking Asus WL500W. It had 2-USb 2.0 ports and was meant to be compatible with DD-WRT. We decided to flash it to DD-WRT but the router still wasn't delivering the performance we needed and we moved on to other brands/platforms. Interesting side note, about the time we gave that router away, we ran across a developer (RMerlin) who worked on WDLXTV a 3rd party firmware for a certain WDTV video player. We had the compatible player, and WDLXTV was an improvement over the stock WD FW. The same player is a bit dated by 2019 standards but still streams video files from a USB drive to one of our HD TVs
In 2011 we bought an RT-N66W which was a big improvement over the WL500W. The next year (2012) we purchased an RT-AC56U. Shortly afterward we found RMerlin had developed the Asuswrt-Merlin FW for Asus routers. Ever since, Asus and Merlin have been inseparable for us; as soon as a new Asus router comes out of the box, we immediately install Merlin's latest build. We've recommended/installed the fork on quite a few Asus routers for our friends, relatives and others. To their credit, Asus has released firmware that is more secure and stable than in previous years. Always use what works best for your needs, and perform your research before attempting to flash firmware.
We like Asus but the Merlin fork has always offered us more than the official FW, with regular security updates and fixes for flaws in the official builds, which enhances the GUI without reinventing the wheel. We'd have moved away from Asus several years ago if Merlin didn't exist. Asus has adopted some of Merlin's features but currently keeps enough of their official firmware as open-source, so the Merlin fork can move forward. Asus now offers AiMesh which Merlin doesn't. Whatever direction Asus takes the brand and their firmware in the future, as long as Merlin is able to continue, we'll continue to use Asus and Merlin. We have an RT-AC3200 and RT-AC-86 in regular service with others as backups. Some alternative FW takes a different approach to official FW, and can have a much steeper learning curve than Merlin. Various sources put the number of folks who have downloaded and use Merlin at between 150K and 200K.
SNB/Asus Wireless forum(s) contains many threads covering many subjects and usually there's a proficient or advanced user who tries to answer questions when they have spare time. Asus official product support has improved in the past year. Hope this helps.