Nike's heal drops to 5 turns when she's Lv65. Though it still only heals 750. Sol's heal is definitely a lot slower, from 1200 HP at 7 turns to 1500 HP at 6 turns at Lv 55, while removing a status ailment/debuff. According to JP wiki, Cassiopeia's heal caps at 1200 and Andromeda 1600, both at 5 turns.
In terms of pure healing, Cassiopeia and Andromeda have much better heal per turn rate (up til turn 21 for Cassiopeia vs Sol, which isn't very common from what I've seen yet, unless you solo a raid boss or something), though unlike Sol, neither provide damage support aside from regular attack unless willing to spend on cross skill. Neither Cass or Andro have status ailment removing skill so taking the skill from Vivian may be ideal for certain fights but giving up your extra slot for other potential skills. You can probably out heal poison but dealing with something like charm is annoying, especially if you're trying to get off 5 bursts chain.
So I'm sure she becomes desirable in future content for free players but as I mentioned before, Andromeda's degree of healing isn't needed to clear any of the current content, it'll make it easier though, that's for sure.