Hello Anna,
No Shellac (or any other gel polish) which is applied correctly by a fully trained nail professional and maintained correctly by the client does not damage your nail beds and/or make your nails brittle.
I have found that when people have said Shellac / Gelish / gel polish etc has damaged their nails it is because they are not following the aftercare advice such as cuticle oil twice daily and more importantly getting their manicure removed by a professional. Some people tend to pick their Shellac or gel polish off themselves which of course will cause damage to the nail bed. To avoid brittle nails stick to the cuticle oil but yes brittle nails can also be caused by other factors.
IBX Repair is a good system to wear under Shellac / gel polish for those who have split, brittle or damaged nails as it helps to really repair the damage and get your nails into a good healthy state.
Hope this helps - bottom line is make sure you goto a nail professional who has trained in Shellac and knows the correct application and removal procedure and follow all aftercare advice!
Kind regards,
Metta
http://www.nailsbymets.com