When it comes to danger and damage of whatsoever origin. :-)

In most cases it is related to either improper implementation and usage or the lack of necessary caution and good care.
This is mostly true for eyelash extensions in all aspects of their usage.
The fear of their detrimental effect on natural eye lashes is based on cases of unprofessional application and poor maintenance and hygiene.

What is of paramount importance for the safety of the extensions fixation is the knowledge. Skills and experience of the technician you will get to do the service.
The basic cause of serious natural lashes damage is the discrepancy between the types of natural lashes and eyelash extensions.

To avoid it, the technician should choose eyelash extensions with such thickness and length to match natural lashes.

If he puts too many or too heavy extensions on thin and sparse lashes, the former will definitely ruin the latter and prevent their regrowth. More than necessary glue is also dangerous since it will clump them together. Furthermore, any failure on his part to put a separate extension on a separate natural lash. Will result in serious consequences for your natural eye lashes.

In view of these requirements, you should refrain from insisting on having very thick or very long eyelash extensions. Because it is not so much the beauty of your look but rather the health of your lashes that counts most.
All the delicacy of the procedure makes it out of the question to even think of applying eyelash extensions by yourself. Or rely on a friend with less or no experience at all. Experimenting with your own lashes is inconceivable and more than risky.

Since hygiene and caution is something you need to stick to consistently. You have to clean the extensions with oil-free products without roughly brushing or rubbing them but gently touching them instead.

In applying make up the rule to follow is to avoid water-proof lashes.

In case of an adequate application and upkeep. The eyelash extensions will gradually shred together with the natural ones. Your concern is to let them do that without any interference by attempting to remove them or pick them off.

Picking the eyelash extensions off may lead to a permanent ruin of the hair follicle, which is responsible for natural hair growth. In that way you may lose your natural eyelashes for ever.

If you want to have eyelash extensions in good condition. You'd better go for a refill on a regular basis to a trustworthy technician. Taking the initiative to put some extensions on your own could be no less harmful than the previous do-it-yourself risky actions.

All in all, eyelash extensions are intended to bring you pleasure and delight from your boosted appearance and self-confidence. What is up to you is only to be careful in your choice of the person to apply them. And the way you maintain them as providers of that pleasure and delight.