Hello sebasvifiz,
I don't have an XP-Pen 15.6 Pro tablet, so I can't attach it directly to the universal mount (Ugreen universal mount) to see if the universal mount can hold and support the XP-Pen 15.6 Pro tablet.
I have a tablet measuring 19.5cm x 28.5cm and weighing 0.8kg. The clamp of the universal holder can hold and carry this tablet without any problems. The clamp of the universal holder has a maximum opening of 21.0cm - therefore this clamp can hold my tablet in height (19.5cm) - but not in width (28.5cm).
If it is true that the XP-Pen 15.6 Pro tablet measures 44.3cm x 28.0cm, then the answer is: no, the universal holder cannot enclose or hold the XP-Pen 15.6 Pro. And if it is true that the XP-Pen 15.6 Pro tablet weighs 3.3kg, then the universal holder would not be able to support the XP-Pen 15.6 Pro tablet - the universal holder would slowly collapse.
My tip: look for a stronger tablet holder.

No, that's too big or too heavy.