I’ll take one decent sized (27” for me) 4K screen over two. Completely solved all problems.
Productivity? I can only focus on one thing at a time. Managing windows and which screens they are on is an overhead I don’t need to deal with.
I get these when I have monitors unbalanced off to one side and have my head constantly turned in that direction. On the other hand I've had 5-wide monitor setups and been fine, because they were properly balanced.
> Managing windows and which screens they are on is an overhead I don’t need to deal with.
Conversely, constantly managing which windows are on top and visible is overhead I don't need to deal with. I use a quad 4k 29" monitor setups at minimum these days, and bought the extra monitors for workplaces that won't provide them. I want documentation, my IDE, my fullscreen game I'm debugging, etc. all visible at once without the need to constantly alt+tab around.
In many cases I yearn for a 5th monitor. Debugging networked stuff, for example, might benefit from two debuggers, two fullscreen games, and then I'm already forced to split monitors if I want documentation. For desktop games I can at least split one monitor between two game windows - although that's not an option for console dev, typically, unless I want to resort to even more fiddly picture-in-picture nonsense...
EDIT: I'll also print off sheet music single-sided so all pages are visible at once - and so I don't have to raise my hands off the instrument to flip pages. Same principle IMO - place stuff in a relatively stable configuration once up front, so it's not constantly interrupting my flow.
So, for example, a primary monitor in front and two secondary monitors to the left can be problematic for me - my main content might be front and center, but constantly looking off to the left only, and not the right, causes problems.
Balance it with another monitor or two on the right - and moving content over there as well - and I won't have any issues, even if I'm still using the leftmost monitor some.
But then, I can cause problems with that same setup - if for example I have only the leftmost monitor(s) connected to a Mac Mini, and end up doing a bunch of iOS/OS X dev/porting/debugging in a row. When I first had problems, I was using Synergy - I stopped doing that as much in favor of a second keyboard/mouse, or shelling into that machine (despite it being on my desk!)
The multi-screen trend is now pursuing its mainstreamification to every offices and home desktops. But at the same time Earth is sending us warnings: we need to slow down. Given the state of our knowledge on that matter, I am surprised that the general consensus on YN is not to have less screens and reserve the resources to better usage.
I find that having different things live on different monitors (and virtual desktops on those monitors) helps me focus on things more easily. E-mail/calendar/Jira is on my side monitor, whereas my main one is filled only with my editor and terminal. It helps because I can _avoid thinking about_ things that aren't on my main screen, until such time as I need a break and deliberately shift focus.
Maybe it's my slightly lowered perception threshold, but having a screen in my peripheral vision where e.g. new emails could be seen wouldn't be useful: I'd still see that a new email has arrived and would still need to decide whether I want to look at it now or whether that can/has to wait. And that's only a small step away from having a popup notification on my main screen about a new email.
When the pandemic started I left a couple of my monitors at the cowork space where I had my desk, and had to do with what I had at home (one 1920 x 1080 and another at 1366 x whatever).
I had another monitor lying around, and eventually I plugged it into my box as well.
As I didn't have a way to keep the monitor in a decent position... I put it as a "console" window, almost on level to the keyboard and right in the space between it and the larger monitor I had.
Eventually I went to pick my stuff from the cowork, so now I have 3 HD monitors arranged in portrait mode side by side; and I still crave for the "console" window.
Although I do reckon I wouldn't mind having a 4K monitor, but the price of those in my country are prohibitive (To the tune of +400 USD last time I checked).
Also I'm looking forward to the point that I can buy a decent VR headset, and then I think I'll be at a point of no return.
I'd love to have 2 or 3, but alas I don't think my MacBook is powerful enough to handle it (even aside from the expense)
I even make use of three screens for Rust dev. One for documentation, one for the editor and one for execution.
I have two 28" 4K external monitors, and both are completel covered in multiple windows for the one thing I am working on right now (well, would be if I wasn't posting this).
It really depends on what your workflow is like and what constitutes a "thing".
Are they windows you need to look at at the same time?
It wasn't my first choice (I was interested in some more expensive BenQ models), but when I bought it the options on amazon (or elsewhere, really) were limited. Still I would buy it again.
edit: also, I called it ultrawide, but while wider (and curvier) than other monitors, it is not actually ultrawide.