Erenia is pretty useful for most situations. I use her for mob clearing. Have a tank behind her and once her skill activates, she clears nearly all of the ones blocked by her and the tanker. Of course I can't speak for the more difficult ones like bosses, though I did pit her against that vampire lord on the previous event and she did pretty well against him with two healers supporting her. Also heard something about her being effective against a dracolich, but I haven't faced one yet, so I wouldn't know. XD
Idk about the other two dragon riders.