I don't think taken out of keyword is the same as overpowered. Silent Ones fill a role. They heal real good and make pillars and can do both from a safe distance. They can fight, and potentially make soulstones, but the arcanists have much better choices for both.
The arcanists have other healing options, but none of them are as good at healing and surviving. Emergency surgery is on relatively fragile characters who need to get up close. With mannequins I used to get around this by having a couple, cause they are cheap in keyword. WP 3 makes them vulnerable though, counteracting that somewhat.
Firebrand burns important resources to heal. This isn't bad, and I still like them, but I get why people might want healing that doesn't cut offensive power. Embrace the flame is good if burning gets out of control or in a pinch. Also, bonus action so.
Myranda is a 9 stone henchman.
So, we have a model that is very good at healing, but (imho) only as good at healing as other 6 stone models are at what they do. You see a lot of the Silent one because, like others have said, healing is useful to many crews and her ability to work at a distance makes her a safer place for magic training. But I don't think they need a nerf as much as some of the competing options need a bit of review.