[Magisk] TheDevFreak:
I've had an issue with magisk on my LG G Watch (W100) running android wearos 2.0 (rom port from g watch r) for a long while now.

You can use su perfectly fine however no modules will load due to what seems to be a mounting problem.

Here is my log from uninstalling then installing 16.6

I:operation_start: 'Flashing'
Installing zip file '/sdcard/Magisk-uninstaller-20180627.zip'
Checking for MD5 file...
Skipping MD5 check: no MD5 file found
I:Zip does not contain SELinux file_contexts file in its root.
I:Legacy property environment initialized.
Archive: /sdcard/Magisk-uninstaller-20180627.zip
inflating: META-INF/com/google/android/update-binary
inflating: META-INF/com/google/android/updater-script
inflating: arm/magisk
inflating: arm/magiskboot
inflating: chromeos/futility
inflating: chromeos/kernel.keyblock
inflating: chromeos/kernel_data_key.vbprivk
inflating: util_functions.sh
inflating: x86/magisk
inflating: x86/magiskboot
inflating: META-INF/MANIFEST.MF
inflating: META-INF/CERT.SF
inflating: META-INF/CERT.RSA
************************
Magisk Uninstaller
************************
- Mounting /system, /vendor
- Device platform: arm
- Found boot/ramdisk image: /dev/block/mmcblk0p15
- Unpacking boot image
MagiskBoot v16.5(1658) (by topjohnwu) - Boot Image Modification Tool
Parsing boot image: [/dev/block/mmcblk0p15]
KERNEL [14321324]
RAMDISK [1549163]
SECOND [0]
EXTRA [0]
PAGESIZE [2048]
NAME []
CMDLINE [androidboot.hardware=dory user_debug=31 maxcpus=4 msm_rtb.filter=0x3F pm_levels.sleep_disabled=1 console=null androidboot.console=null]
CHECKSUM [d4ac5e08b7ae86f9a3890211abde198f77ae0635]
DTB [9261684]
KERNEL_FMT [raw]
RAMDISK_FMT [gzip]
- Checking ramdisk status
MagiskBoot v16.5(1658) (by topjohnwu) - Boot Image Modification Tool
Loading cpio: [ramdisk.cpio]
- Magisk patched image detected
- Restoring stock boot image
sh: -f: unknown operand
- Removing Magisk files
resetprop: Initialize error
- Unmounting partitions
- Done
I:Updater process ended with RC=0
I:Legacy property environment disabled.
Updating partition details...
E:Unable to statfs '/system/.'
df: /system: Permission denied
df /system > /tmp/dfoutput.txt process ended with ERROR: 1
I:Data backup size is 651MB, free: 2038MB.
...done
I:Set page: 'flash_done'
I:operation_end - status=0
I:Set page: 'clear_vars'
I:Set page: 'main2'
I:Set page: 'install_type'
I:Set page: 'install'
I:Set page: 'flash_queue'
I:Set page: 'flash_confirm'
I:Set page: 'flash_zip'
I:operation_start: 'Flashing'
Installing zip file '/sdcard/Magisk-v16.6.zip'
Checking for MD5 file...
Skipping MD5 check: no MD5 file found
I:Zip does not contain SELinux file_contexts file in its root.
I:Legacy property environment initialized.
Archive: /sdcard/Magisk-v16.6.zip
inflating: META-INF/com/google/android/update-binary
inflating: META-INF/com/google/android/updater-script
inflating: arm/magiskboot
inflating: arm/magiskinit
inflating: chromeos/futility
inflating: chromeos/kernel.keyblock
inflating: chromeos/kernel_data_key.vbprivk
inflating: common/addon.d.sh
inflating: common/boot_patch.sh
inflating: common/magisk.apk
inflating: common/util_functions.sh
inflating: x86/magiskboot
inflating: x86/magiskinit
inflating: META-INF/MANIFEST.MF
inflating: META-INF/CERT.SF
inflating: META-INF/CERT.RSA
************************
* Magisk v16.6 Installer
************************
- Mounting /system, /vendor
- Target image: /dev/block/mmcblk0p15
- Device platform: arm
- Constructing environment
Invalid image: not signed
- Unpacking boot image
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
Parsing boot image: [/dev/block/mmcblk0p15]
KERNEL [14321324]
RAMDISK [1273620]
SECOND [0]
EXTRA [0]
PAGESIZE [2048]
NAME []
CMDLINE [androidboot.hardware=dory user_debug=31 maxcpus=4 msm_rtb.filter=0x3F pm_levels.sleep_disabled=1 console=null androidboot.console=null]
CHECKSUM [d4ac5e08b7ae86f9a3890211abde198f77ae0635]
DTB [9261684]
KERNEL_FMT [raw]
RAMDISK_FMT [gzip]
- Checking ramdisk status
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
Loading cpio: [ramdisk.cpio]
- Stock boot image detected
- Backing up stock boot image
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
Compressing to [stock_boot_07db08981a798e06b78c98a7f79ba0bb178d81a9.img.gz]
- Patching ramdisk
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
Loading cpio: [ramdisk.cpio]
Add entry [init] (0750)
Patch with flag KEEPVERITY=[false] KEEPFORCEENCRYPT=[false]
Save SHA1: [07db08981a798e06b78c98a7f79ba0bb178d81a9] -> [.backup/.sha1]
Loading cpio: [ramdisk.cpio.orig]
Backup mismatch entry: [init] -> [.backup/init]
Dump cpio: [ramdisk.cpio]
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
Loading dtbs from [dtb]
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
- Repacking boot image
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
Parsing boot image: [/dev/block/mmcblk0p15]
KERNEL [14321324]
RAMDISK [1273620]
SECOND [0]
EXTRA [0]
PAGESIZE [2048]
NAME []
CMDLINE [androidboot.hardware=dory user_debug=31 maxcpus=4 msm_rtb.filter=0x3F pm_levels.sleep_disabled=1 console=null androidboot.console=null]
CHECKSUM [d4ac5e08b7ae86f9a3890211abde198f77ae0635]
DTB [9261684]
KERNEL_FMT [raw]
RAMDISK_FMT [gzip]
Repack to boot image: [new-boot.img]
KERNEL [14321324]
RAMDISK [1584459]
SECOND [0]
EXTRA [0]
PAGESIZE [2048]
NAME []
CMDLINE [androidboot.hardware=dory user_debug=31 maxcpus=4 msm_rtb.filter=0x3F pm_levels.sleep_disabled=1 console=null androidboot.console=null]
CHECKSUM [6c4ec6b1b3c9b69612aeb7c7c1c4e0a69c9694a8]
MagiskBoot v16.6(1662) (by topjohnwu) - Boot Image Modification Tool
Cleaning up...
- Flashing new boot image
- Unmounting partitions
- Done
I:Updater process ended with RC=0
I:Legacy property environment disabled.
Updating partition details...
E:Unable to statfs '/system/.'
df: /system: Permission denied
df /system > /tmp/dfoutput.txt process ended with ERROR: 1
I:Data backup size is 655MB, free: 2030MB.
...done
I:Set page: 'flash_done'
I:operation_end - status=0

and here is my /cache/magisk.log

07-08 10:52:57.363 165 165 I Magisk : ** Initializing Magisk
07-08 10:52:57.364 165 165 I Magisk : * Creating /sbin overlay
07-08 10:52:57.371 165 165 I Magisk : * Mounting mirrors
07-08 10:52:57.372 165 165 E Magisk : mount /dev/block/platform/msm_sdcc.1/by-name/system->/sbin/.core/mirror/system failed with 16: Device or resource busy
07-08 10:52:57.372 165 165 I Magisk : mount: /sbin/.core/mirror/system
07-08 10:52:57.372 165 165 I Magisk : link: /sbin/.core/mirror/vendor
07-08 10:52:57.372 165 165 I Magisk : bind_mount: /sbin/.core/mirror/bin
07-08 10:52:57.373 165 165 I Magisk : * Setting up internal busybox
07-08 10:52:57.511 172 172 I Magisk : Magisk v16.6(1662) logger started
07-08 10:52:57.512 168 168 I Magisk : Magisk v16.6(1662) daemon started
07-08 10:52:57.518 168 174 I Magisk : ** post-fs-data mode running
07-08 10:52:57.519 168 174 I Magisk : Create /data/adb/magisk.img with size 64M
07-08 10:52:57.591 168 174 I Magisk : * Mounting /data/adb/magisk.img
07-08 10:52:58.112 168 174 I Magisk : * Running post-fs-data.d scripts
07-08 10:52:58.112 168 174 I Magisk : * Running module post-fs-data scripts
07-08 10:52:58.112 168 174 I Magisk : * Loading modules
07-08 10:52:58.114 168 191 I Magisk : * Starting MagiskHide
07-08 10:52:58.114 168 191 I Magisk : hide_utils: Hiding sensitive props
07-08 10:52:58.114 168 191 I Magisk : hide_list add: [com.google.android.gms.unstable]
07-08 10:52:59.608 168 235 I Magisk : ** late_start service mode running
07-08 10:52:59.608 168 235 I Magisk : * Running service.d scripts
07-08 10:52:59.609 168 235 I Magisk : * Running module service scripts
07-08 10:54:30.053 168 191 I Magisk : proc_monitor: com.google.android.gms.unstable

I think the key issue here is:

07-08 10:52:57.372 165 165 E Magisk : mount /dev/block/platform/msm_sdcc.1/by-name/system->/sbin/.core/mirror/system failed with 16: Device or resource busy