I have only ever seen one in person once, and that was likely 30 years ago; they are quite sweet! I do not know how much handier they might be than an ordinary universal such as the #2 B&S that I have, I guess that one thing would be that you could mill an angled spiral with a parallel end mill. A disadvantage would be the shorter table compared to a similar size plain or universal mill. I don't think I need one, but it sure would have bragging rights to own one.
A well dressed Maho (or Deckel, or . . . any number of makers) has similar moves / features as the Omniversal. Yes, the set up time can be lengthy and requires attention to detail (like most machining tasks). One drawback of such machines (assuming you can track down the attachments) is that they are generally quite small, for the amount of metal and floor space they take up. It is fair to say that every mill is different and the Omniversal likely has some movements that are not available on others.
Sometimes the movements seem pretty academic. For example, the early Maho has the ability to roll the vertical head, in a smooth controlled manner, while milling. I have yet to conjure up why such a geometry would have merit - but many of these old machines had some pretty unique features.