Hi florjafox & South Jersey Styx,
the only thing I could see would be throw rugs affecting the urethane premixed flexible grout.
maybe trapping some moisture under them and not allowing them to breath.
but I didn't see anything on the Durafloor Data sheet and I read them since I'm in the Tile business.
http://www.congoleum.com/pdf/duraceramic_install.pdf
I'm not to particularly keen of these type of tile floors but they do look nice when done correctly.