I've been playing DF on my steamdeck for a while and I have thoughts.
At a basic level, is it playable? Yes, very much so. The new UI is very mouse driven, so using the touchpads to control most things has been very easy. I had to map the back buttons to zoom, because I can't handle the game at its default zoomed in level, especially when scrolling around and trying to fit a larger room all on screen at once.
The UI is a little on the small side. It's fine for me for readability and such, but it does become a bit straining over a longer play session. And messing with the settings didn't affect it at all.
There were some issues that seemed to be either deck specific or small/extra wide screen specific. On was that on the animals screen, the pet/butcher/geld buttons hang outside the window and register excessive clicks, often marking and unmarking an animal for slaughter like 10 times. So you just click and hope it stops on what you want. Similarly, some menu scroll bars were misaligned and wanted to scroll to a different place than where your mouse would be pointing.
Most frustratingly, the entire intelligence section of justice just didn't work. None of the tabs or reports were clickable.
I've moved over to the big PC, because it's easier on the eyes and fingers. But overall, it works dangerously well. I.e., it's easy to be like "I'll just zone some bedrooms before I got sleep" and suddenly it's 2am and you're making sure the fortress guard all have red capes.