Android device not listed in adb devices

Question

I have this super cheap JAY-tech Tablet PC PA7807-8 (FYI not exactly the linked model) and no matter what I do I couldn't get it recognized by adb/fastboot, meaning it doesn't appear in adb devices or fastboot devices at all. It has Android 4.4.2 installed (which can't be updated). I'm using adb 1.0.31 on an up to date Debian 8. I have:

• enabled 'Developer options' and therein 'USB debugging'; thereafter the device displays the "USB debugging connected" notification when the cable is in,
• tried all connection types like MTP, PTP, and "USB large storage",
• checked the idVendor attribute of the USB device (it's 1f3a) and added the line SUBSYSTEM=="usb", ATTR{idVendor}=="1f3a", MODE="0666", OWNER="taylan" to /etc/udev/rules.d/99-android.rules after which I rebooted Debian,
• created ~/.android/adb_usb.ini and added the line 0x1f3a,
• restarted the adb server, rebooted/unplugged/replugged the device,
• tried running adb kill-server/start-server/devices as root.

Am I missing any possibilities? My Nexus 5 works just fine with adb. Are there Android devices that flat out don't support adb?

Edit: by the way, when I set the connection mode to "USB large storage", /dev/sdb and /dev/sdc pop up but neither can be mounted (says "no medium found on sdx" when trying to mount directly, and sdb1 and such don't pop up); when I set it to MTP, jmtpfs still says it couldn't find any MTP devices.

Answer 1

If your Nexus works with ADB the problem is with your JAY-Tech.
I've learned the hard way that there are lots of devices that flat-out won't work with ADB.
Nexus devices are the only sure bet.