In Judo the aim when applying nage is to land your 'opponent' on their back to score your full ippon (winning technique)and therefore your ukemi tend to be either fully backwards or slightly to one side. Causing someone to fall on their front scores nothing (if the referee is awake) so being thrown to the front should not be an issue so long as your partner is reasonably skilled and can control the direction of the throw. With a lifetime experience of Judo I voted for forwards being the most dangerous, but of course vertical is really the worst. I have practiced Aikido of varied styles which range from soft to hard and I am surprised that although the practice of ukemi is given great lip service the practice of ukemiwaza in the main seems to be neglected with beginners expected to pick up the techniques from sometimes painfull experience. Ukemiwaza are part of a normal Judo practice session with techniques to the front, sides, rear and forwards rolling. Even high grades will in general practice a full range ukemiwaza as part of the normal warm up session.
I am sure that when ukemiwaza becomes fluid and instinctive through repetition the risk of injury in falling in any direction is greatly reduced.