Yes they are very safe i've had them for 10 years - and they wont damage your nails but the electric file they use one your nail bed before application will.
Since everybody is different, you can't get anyone's advice on this. I myself had them on a little over a year then suddenly developed an allergy...the skin around my nails blistered, bled, then turned yellow and peeled. When I got the nails off, there was so much damage to my nails that they couldn't touch me any longer. That was 6 months ago and I'm still healing from it...
Gel nails are natural-looking enhancement of the nails. They can be used for natural nail overlays, tip overlays, and to help encapsulate a damaged free-edge as it grows out. If you have a personal history of allergies, try to get a clearance from your doctor first to avoid any allergic reactions.
Take care and regards.