I've just been swinging the DJI3 thing round a bit and had no problems
I've been using with S9Plus previously, that was fine. the S21U is a bit longer and heavier obviously but doesn't appear to be a problem for it. I've been using it on a tripod for zoom meetings an hour or more, and it's kept its balance.
As mentioned above, I think the software is designed for iclones, and android as an afterthought. The zoom control on the device doesn't work but the joystick does. The auto track works well if you are on your own for selfie video. I use the software to calibrate the device and change speed of reaction to tilt etc, then use Samsung's own camera to take video. (the camera zoom button on dji is useless)
Charging phone whilst using from the gimble doesn't work as it unbalances it.
I bought new last year for £100, but disappointed with the app.
If you can find one on ebay really cheap and prepared to work with its problems, then its an amazing bit of kit as a straight forward gimble. You can run and jump pan slowly etc and the mobile just floats in the air.
I'm keeping mine as it fulfills my needs.
If I was buying again, I would look for one that works better with android.