I own 2 wndr3700v4, i use them since years and i am very happy.
Few weeks ago, I had no more free space on the flash and I was not able to install any new packages.
To extend the flash free space, there are 2 solutions :
- extroot 
- patch the kernel to enable the unused space  
Patching kernel is not easy, so I chose the extroot solution.
The extroot solution works fine, but in time i am facing with an unsolvable issue : usb storage randomly disconnects, the root fs is lost and the device become non functional until I reboot it.
On last month, here are the disconnected events :
Feb 23 17:17:13 Lede kernel: [759377.797823] usb 1-1: USB disconnect, device number 2
Feb 23 17:34:01 Lede kernel: [760385.463574] usb 1-1: USB disconnect, device number 3
Feb 12 09:02:40 Lede kernel: [898877.228779] usb 1-1: USB disconnect, device number 2
Feb 13 08:40:42 Lede kernel: [983959.407466] usb 1-1: USB disconnect, device number 3
Feb 14 21:08:17 Lede kernel: [81544.189888] usb 1-1: USB disconnect, device number 2
Jan 27 00:22:10 10.0.0.253 kernel: [ 944.660000] usb 1-1: USB disconnect, device number 2
I tested on my 2 wndr3700v4, with several USB keys, the behavior was the same each time. I gave up, and now I'm patching the kernel to use the wasted free space.
Any other feedback about extroot and wndr3700v4 ? am I alone to see this issue ?