Mickey's Toontown at Disneyland and TDL uses painted flats in the "downtown" are to hide the taller roofs of the Gag Factory store and Roger Rabbit's Cartoon Spin ride. They're the maroon and grey skyline poking up above the dimensional facades seen here:
View attachment 395013
In this scenario it works fine, mostly because they're only visible from a few very specific locations where the eye isn't naturally drawn to those parts of the structure (or even the structures in general), and because the land's cartoonish aesthetic allows a departure from realism. Additionally, the land at DL is largely surrounded by layered flats of hills that are used to obscure the iasw show building, TDA parking structure, and other backstage facilities, so the flat buildings fit the general atmosphere of the area.
It remains to be seen how effective painted flats will be in Epcot, where they will be more visible on the main approach to the new attraction, and in an area that strives for a higher level of realism. I fear that it may be similar to the layered castle ramparts in the Fantasyland courtyard at Magic Kingdom and TDL, which helps minimize the scale of some of the buildings, but does little to convince guests it's anything other than a generic warehouse structure.
View attachment 395017
I'm also curious what they'll do about the building's southern side, which will be plainly visible from the Skyliner. It's currently covered in green pre-fab panels (indicating no plans for a surface treatment), and there doesn't appear to be space for a substantial landscaping buffer between it and the service road