When talking about security regarding Asuswrt vs Asuswrt-Merlin, it's mostly what we call in French "une partie de saute-mouton". Basically, in a typical timeline:
1) I find a security flaw. I fix it in my firmware. My firmware is more secure.
2) Asus fixes it in their firmware, and issues an update. Both firmwares are just as secure.
3) Asus fixes a new security issue, and releases an update. Their firmware is more secure.
4) I merge their code into mine, and release a new firmware. Both are just as secure.
So when people ask me which of us is more secure, it's impossible to answer. Whoever issued the last release is the most secure at that precise point in time.
Now, there are a few extra security enhancement that my firmware has which never made it into the stock firmware. But these are just minor improvements, they aren't major security flaws waiting to be exploited. For instance, I use stricter filesystem permissions on some of the system files. Call it a small layer of security hardening, but nothing major to worry about.
In general, the stock firmware has greatly improved in terms of security. Asus are generally doing a very solid job when people contact them with security flaws they have discovered, communicating with the reporter, and taking steps to fix things in a reasonable timeframe. Of all the non-pro-summer router manufacturers, I consider them, today, as one of the best in that area.