Maybe karin is not the best healer for team but she is mainly used for her poison damage, sai on the other hand is most likely used in Strong approaching / Ranked / Survival Trial, etc. and if you want to run a healer in team use kabuto.

Kurenai is more of a support + shield and kabuto is a full healer, they both are different and if you can link Kurenai's chase with your team, you should use her IMO.