Sew-ins are installed by either braiding or twisting the clients natural hair tightly to their scalp (depending on which method the stylist uses). If the braids aren't tight, the hair that is sewn in will not stay in tact (which is an obvious waste of time for the stylist and waste of money for the client). Unfortunately this is where the damage begins. Sew-ins are a lose-lose situation. I am currently dealing with numerous clients that have had sew-ins destroy their hair and are now suffering from traction alopecia (caused by the natural hair being rolled or braided too tightly, OR resulting from that heavy weft of hair causing continual stress and strain on the natural hair).
In all honesty, sew-ins are NOT healthy for your hair. They can't be washed/dried properly, and mildew will begin to grow once they get wet. Your hair will begin to smell after only a few days (I am speaking from first hand experience), and can result in mold and mildew growing. Bottom line, sew-ins are uncleanly and will destroy your hair, and I'm sure any honest/educated stylists will attest to this. Sew-ins are mainly used in the fashion industry to create large, whimsical displays of fantasy hairstyles. Whether the stylist is a licensed cosmetologist, hair braider, or extension artist, this method of extensions is unethical to be used as a semi-permanent extension option, in every aspect.