I have a Samsung phone with the DeX desktop mode. While DeX is a neat feature, it does not feel mature despite being the oldest and most common Android desktop mode. After my initial enthusiasm I do not use it all that much any more.
One reason is that the apps are still the same apps you have normally on the phone, e.g. your regular mobile browser, your regular mobile banking app - and often those do not adapt so well to desktop mode: weird layouts, functionalities that keep assuming fingers on a touchscreen and don’t understand the mouse. Browsers tend to present the phone version of websites, blown up to monitor size. This can usually be changed in a setting, but then the website assumes you are running “proper” Windows/Linux/MacOS, which you are not, and you get weird little issues. Firefox, normally my preferred browser, is just terrible in DeX, so I have to use something else, which means it will not be in sync with the Firefoxes I use everywhere else.
There are also all sorts of little issues, like right-click and copy&paste across apps does not work reliably - because in Android it is very much the app developers’ responsibility to implement support for this, and they often don’t as so few people use DeX.
DeX is also demanding on the hardware. The phone may get warm, which is not great for the battery. Also, my phone has 6GB RAM, and I often hit the limit. That is, Android pretends there is no limit, but this does not work so well. E.g. I am writing something in Office, open the browser to check something, go back to Office and find my work is gone because Android offloaded/reloaded the app to free up memory for the browser. The Mudita Kompakt has just 3GB RAM.
There is also the whole practicality, what are the actual use cases. When travelling it may seem attractive to think, “hey, I can just travel with my phone and leave the laptop!” But if you then carry a portable screen and a keyboard, there is little difference in practice to carrying a laptop. Some DeX-diehards even get a lapdock (an “empty” laptop without the actual computer, just screen&keyboard&battery), which is probably the most comfortable use of DeX on the move - but they are suprisingly expensive for what they are and of course sized like a laptop.
I already have a desktop computer and a laptop, and they are both better at “desktopping” than DeX, and the laptop is as easy to transport as a mobile DeX-setup. So I do not really use DeX much any more. With Google getting serious(?) about the desktop mode there is some hope that we will see technical improvements, more widespread use and better docking solutions. E.g. I would love a mature DeX with fully functional desktop apps, hooked up to a folding portable monitor, say a double-folding 12" monitor that folds down into a smartphone-sized footprint. But the tech is not there yet…