Actually the larger models such as the Ovni 455 and more recently the Ovni 495 and the Ovni 445 are equipped with twin rudders. One drawback though, a bow-thruster is required on such twin rudder boats for manoeuvers within busy harbors. With perhaps the exception of the Ovni 445 whose twin rudders are very close to each other. But i can't really tell since i havent sailed one yet; and being a recent model haven't seen much feedback either on the OvniClub or in the nautical magazines so far.
I have personnaly sailed an Ovni 435 from les Sables d'Olonne down to the Med and can tell the single rudder works very well. If you take the bottom part away i doubt the boat would still be easy to stir
Problems have been reported with the corrosion of the main bar of the hydraulic cylinder on some rather recent models (years 2000 - 2004) where there is no electric continuity from the bottom part to the main part of the rudder, hence causing some problems to lift the rudder hydraulically (still possible with a rope). Once the 2 parts are grounded this is not an issue anymore (see the various threads on this subject). Haven't heard pbs with older models, and until Alubat made some changes, we believe, to some of the Ertalon (polymer) parts in between the 2 rudder elements.
Re lifting keel boats (e.g. Beneteau) which haven't been really designed for this and where the lifting keel is just an option, i understand these are rather poor compromises and hence it is preferable to stick to a fixed keel, taking for instance the short keel version. Re true lifting keel sailing boats the Ovnis and more generally several aluminum boats have been designed entirely for this (see importance of dynamic stability of the hull on such boats); which is not the case of most polyester boats, with perhaps a few exceptions incl. Southerly which have had a rather good swing keel mechanism for a while.