When using a gigantic screen, Plasma notifications may be missed because they can appear out of the user's peripheral vision
We have received upstream complaints about this too. See for example https://bugs.kde.org/show_bug.cgi?id=420541. I turned that bug report into a change only to critical notifications, but I think the issue is still generally relevant to people with huge or ultrawide screens.
The issue here may be conceptual: Our notifications are designed to be visually unobtrusive--even ignorable--but then we use notifications for all sorts of things that really shouldn't be ignored, like job progress, saving a new screenshot, and so on. We may have to rethink this and commit to notifications being a bit more visually attention-getting. However that needs to be carefully balanced with the more common use case of having a single small screen.