I'm using the latest RMerlin 374.40 firmware for over 4 days with no issues.
Just a basic user here (wired/wireless LAN and internet), but this is the most stable version yet.
This is the recommended version for all Asus routers except for the RT-AC68U (for stable operation with that model you want 374.40 alpha 4).
Very fast and responsive and the range is as good as any of the old -em versions or before.
Make sure you do a full reset to defaults and power cycle the router a couple of times. Also, use a new ssid or 'forget' the old network on all your clients, then re-associate them for the best performance and stability.
I have tried many firmware versions since I bought my RT-N66U almost a year ago; this is easily the best. The most secure, the most stable, the most features and the highest performance too.
I think not upgrading to the latest firmware (especially an RMerlin version) is doing the hardware a disservice and possibly putting the security of the network at risk too.