The consistency of the theming isn’t the issue, it’s that it’s just theming (unless I’m just misunderstanding). KDE or GNOME with an XP theme applied settings toggled still acts like KDE or GNOME rather than acting like XP. The resemblance is skin-deep.

Good theming is great to have, but what’s more important is that the user’s prior experience and muscle memory still applies, e.g. the task manager can be summoned in the same ways, settings panels are structured similarly (and aren’t either overflowing or too stripped down like KDE and GNOME, respectively), key shortcuts are the same with no caveats, etc.