When applied correctly in terms of weight that your lashes can hold, then they are safe to your lashes.
Repeated use of eyelash extensions that are too heavy (meaning too thick and or long ) can cause a condition called traction alopecia, where the hair falls out due to tension put on the hair shaft.
This can damage the hair follicle which can slow down or is some circumstances stop the production of hair. This can lead to a vicious cycle where women feel the need for ongoing lash extensions to boost the little natural lashes they have left resulting in even more hair loss.
A good indication that your extensions are too heavy is if they are droopy, falling off after about 1 week or turning sideways on you.