Flower memories (especially 6* memories) are great. The small stat boost is still a welcome boost (I think it's affected by ability stat buff unlike equipement boost) and the additional ability is awesome. It may seems weak but think it as an ability you can put on every girl to customise her. With 5 memories per team, you can get pretty broken stuff. Like, I really recommend using five fully unlock "rainbox pot" silver memories. It gives +15 ATK for a 300% solar attack. It's easy to get and gives a solid +75% ATK for everyone once you used your sun.
Flower memories are pretty underestimated but really start to be good when you can get fully unlocked 6* memories. 5* are mostly not good and 4* are trash.

Pavonia is really great but I think she is a bit too frail for really hard bosses. I tried her but didn't perform better than some others (or maybe full offense is not really my style). But for the regular stuff, I like her.
She really shines on the "test your strengh" part of raid boss events with good team mates. You can reach stupid amounts of trillions damages. It's totally useless and overkill though but it's very satisfying.
But you can only get her with 300 Rainbow Medals when dhe is available in the shop. She was added on September 20. Who know's when she will came back. On the good side, it will let you build up Rainbow Medals.