Starting with Asus's 22.214.171.124.380_3000 and upward, a new firmware format is used. To flash Asuswrt-Merlin on one of these newer firmwares, you will need 380.61 or newer. Likewise, downgrading to an older version is not supported (try firmware recovery mode if you really need to downgrade)|
It's recommended to reboot the router BEFORE flashing it, to free up sufficient memory. Otherwise, there's a chance the router might reject the firmware, due to running too low of free RAM.
If coming from a non-Asuswrt firmware, you will have to do a factory default reset.
While it is generally not necessary to restore to factory defaults, it's not a bad idea, especially if there is a big jump in version number (from 112 to 178 for example). No need to do the 30/30/30 dance as required by DD-WRT - just do a plain Factory Default reset, or turn the device on while keeping the WPS button pressed (procedure can be different from one model to another).
It is very hard to brick an Asus router. If something goes wrong during flashing, you can put your router in Recovery mode by powering it on while you keep Reset pressed. After your release it, the power LED will either blink or stay off (depending on the model). At that point, you can either access it through http://192.168.1.1 (make sure you first set your PC on a static IP within the same range, i.e. 192.168.1.100 for example), or through the Firmware Recovery Tool provided on Asus's support CD. You will then be able to flash a working firmware.
If something looks weird, don't waste too much time: save your settings, reset to factory default, reconfigure the basics, and see if the issue is resolved. If not, you can always restore your saved settings, and do some more advanced troubleshooting.
It is NOT recommended to restore settings saved under a different firmware version. It might work, but there is no guarantee.