I've been using a Steam Deck as my only PC for almost a year now, for work (graphics design, web dev, illustration, some Blender) as well as play ofc. Aside from my suboptimal dock options (Valve doesn't sell any hardware in my market) It's been a very smooth experience, and I've not even had to disable immutability at any point.
I would like to support the point that Game Mode is one of the most important features the Deck has, and losing out on it by installing Ubuntu feels like a loss.
But I would also like to note that Steam OS now has Distrobox built in: for most use-cases you can just set up all the software you need inside an Ubuntu container without much hassle.
Ultimately though, the form factor is the main difference. If I only needed to keep it docked all the time a Deck would not make much sense. But I love shifting to my bed after a workday and playing anything and everything I would have needed to sit at a desk to do before!