I have round air vents... I don't know if I'm making a mistake in the installation... but it doesn't stay stable at all... I don't know what's wrong. I tighten it as much as possible to wedge it in... but nothing... it slips and falls off.

My mobile phone is heavy (310 grams including the case and screen protector) and I was worried that wherever I put it, there was a risk of breaking the support point in the car during a vibration, while at the same time I wanted something discreet and sturdy.

After doing some research, this specific mobile phone holder was the only one in terms of the way it is supported, as you can place it on the VENTS of the car, a point that can withstand more forces in potential vibrations. Also, you can't even see it if you remove the phone.

I also liked another one that attached to the cup holder, quite expensive, but I rejected it because in the car those are close to the lazy person, so I thought it wouldn't be convenient because I would have to lower my gaze frequently while driving (I think it's more suitable for cars with cup holders closer to the dashboard, and it's also not discreet).

However, with a heavy mobile phone, I think these two were the best options. If my mobile phone was lighter, there would be other choices, regardless of the fact that I might still end up with this one.

The support mechanisms of the holder are with strong springs, both for the vent and for the phone (hooks), while the risk of damage to them is limited due to the padding at the support points.

To put the phone in the holder, you need both hands, which means it secures well, but on the other hand, it may be a bit more difficult to use compared to others, where you only need one hand for this task. It is easier to remove.

It remains to be seen if it keeps the phone in place without occasionally coming loose, but I have the impression that this is unlikely to happen. I think it has to do (besides the strength of the vibrations) with the size of the device (larger devices would make the hooks tighten more) and its weight.
I cannot predict whether the horizontal or vertical position is more dangerous for such an occurrence. Maybe in the horizontal position, the bottom hook receives greater pressure, while in the vertical arrangement, the forces are more balanced. I will have the device in a vertical position.
The holder seems to be stable. We will see... The purchase is recent.
If my phone or the entire holder with the phone falls, I will come back to it.

The phone rotates on a threaded ball (horizontally and vertically), and it also allows for changing the tilt angle, which is more than enough, even though the phone is very close to the vent.

I feel that the product is of very good quality (I have also bought a leather steering wheel cover from the same brand - excellent!), and I am satisfied with the purchase.