According to Avalon's description and dialogues, she is meant to be the strongest vessel. However, in practice I find her to be more in line with mid tier vessels like lamia and wild lamia. Sheat mode seems to be significantly weaker to simply having the other vessel assisting from outside, while sword mode is only a small buff for a few turns (enervate considered).

She is a nice back line supporter, but I am not able to justify using her in maxed out teams. Am I missing something? 

She can safely carry allied vessel's in her stomach which restores their MP each turn ( heals them too i think) and also when she has an ally inside her she can borrow there MP to use her Excalibur attack which can do decent damage.