In my experience (not big productions) all PC playback problems were related with the DVD playback software, and more specifically with customers trying to use Windows Media Player as DVD player.
Have you personnaly saw one of those machines? I mean, have you "see" by yourself a computer that can play comercial pressed DVDs and not your DVD-R discs?
You can ask to a confident customre with such problems to install the free and crossplataform
VLC Player to check if he can play your disc using this software.
An after all your authoring/burning workflow can affect your discs, i.e. are you using labels? What brand of media are you using? What burning speed
are you using? . . . and so, and so.
Hope that helps !
Alberto