Is a butt augmentation using implants safe and common?

See details: If it is done by a competent plastic surgeon it is safe but it is not commonly done.

Less common...: ...Compared to fat transfer, which is really the best option for all but the skinniest people. Relatively safe if done by an expert, but complications can certainly occur.

Yes: Gluteal implants are an excellent option for thinner patients wanting a more curvaceous derrière. They are safe when performed by a well trained surgeon although they still carry a higher complication rate than breast implants. Sientra, the manufacturer, will only sell them to plastic surgeons rather than any surgeon to try and minimize complications.