busybox doesn't work on marshmallow

Question

The system builtin ip command works correctly with:

# ip route show table wlan0
xxx.xxx.xxx.xxx/xx dev wlan0  proto static  scope link

But busybox ip doesn't:

# busybox ip route show table wlan0
ip: invalid argument 'wlan0' to 'table'

Looks like busybox failed to read some system settings.

1. So what's wrong here?

2. When tried to uninstall, the Busybox Free app froze. Is it safe to manually remove busybox and its symlinks in adb shell?