Its a bit of both
Filing, you need to know how to file and most people go in too harsh - I had a friend who had bleeding nail beds from filing too much
Also gels must not be cured to the skin and if you dont have clean nail beds, cuticles removed, side walls smoothed back, and apply the gels as thinly as possible, you are going to cure it to your skin
Not capping, curing to the skin, causes the gel to lift - water gets in under, and there you have a breeding ground and before you know it - allergies
An allergy to gel can cause you problems with dentistry and hip/knee replacements going forward . A gel allergy is not something easily recovered from
Most home users starting out dont even know what a cuticle is.
I use home kits , I also use some professional gels ( so much easier to work with but you usually have to show your qualifications to buy ) , and I learned the hard way and have invested in learning how to care for my nails
Right now Im back working in a kitchen and my nails are a short mess - I usually prefer real nail talons. However because of my job, I have to be careful with what I wear. I cant have something liable to chip or melt or pop off - so my press ons are out, wraps completely out ( they tend to melt on the cut edge ) and because they are short at the moment dips ( the stongest and longest lasting ) which will be the next thing on , I cant apply at the moment cos of breaks and splits, Im going through the stage of growing length and strength because I refuse now to cover imperfect nails
But I do that, every now and then go bare. First week or two I sang and split, then before I know it they are back rock hard and strong