Rescaling between two monitors that are extending each other, even with different scales works well, its when one get disconnected and the window is forced to move to another screen that causes issues.
As to legacy software, most software works fine. Those that never cared about scaling are handled by the system easily enough. It's those rare cases where the legacy software is saying they support scaling to solve some issues with them not using the windows apis but not really doing anything to actually support scaling, such as GIMP.
Those are rare though, and usually are software with it's own rendering stack that don't use the windows apis.
I meant when moving an application window from one screen to another. I saw some issues that didn’t exist when I worked only on the MBP screen, but when I connected my 27” external display, they were suddenly issues noticeable when dragging windows.
As to legacy software, most software works fine. Those that never cared about scaling are handled by the system easily enough. It's those rare cases where the legacy software is saying they support scaling to solve some issues with them not using the windows apis but not really doing anything to actually support scaling, such as GIMP.
Those are rare though, and usually are software with it's own rendering stack that don't use the windows apis.