adb shell su works but adb root does not

By design adb root command works in development builds only (i.e. eng and userdebug which have ro.debuggable=1 by default). So to enable the adb root command on your otherwise rooted device just add the ro.debuggable=1 line to one of the following files:

/system/build.prop/system/default.prop/data/local.prop

If you want adb shell to start as root by default - then add ro.secure=0 as well.

Alternatively you could use modified adbd binary (which does not check for ro.debuggable)

From https://android.googlesource.com/platform/system/core/+/master/adb/daemon/main.cpp

#if defined(ALLOW_ADBD_ROOT)// The properties that affect `adb root` and `adb unroot` are ro.secure and// ro.debuggable. In this context the names don't make the expected behavior// particularly obvious.//// ro.debuggable://   Allowed to become root, but not necessarily the default. Set to 1 on//   eng and userdebug builds.//// ro.secure://   Drop privileges by default. Set to 1 on userdebug and user builds.

In some developer-friendly ROMs you could just enable Root Access in Settings > Developer option > Root access. After that adb root becomes available. Unfortunately it does not work for most stock ROMs on the market.

I ran into this issue when trying to root the emulator, I found out it was because I was running the Nexus 5x emulator which had Google Play on it. Created a different emulator that didn't have google play and adb root will root the device for you. Hope this helps someone.