I have extensive experience of boarding schools.
The honest answer is that it is truly mixed. For children that have outsourced parenting from the start with a nannies and the rest, for those children it will make very little difference. Family connection has never been there, those children have learnt to live life as part of a herd. In some ways boarding is a blessing, they have perhaps more attention there than they otherwise would have at home. There are homes that border on negligence due to lack of time and commitments.
Then we have the children that come from very loving homes, but parents want the best schooling for their children, and depending on whether the school suits them or not. Some thrive and enjoy the independence, others sink without a trace.
If really depends, like everything on the character of the child. If you have a sensitive loving child they are going to find it (very) hard, if you have a child that loves to socialise all of the time, lives for their friends then they will be better suited.
The first few months are horrendous for everyone, and the homesickness is acute for most children.
In today's world with mental health issues, anxiety, self harming, social anxieties and eating disorders, issues on social media it does make me wonder how any parent has the confidence to send their children away to school knowing the avalanche of challenges they face as a normal teenager in 2021. Even the most resilient can quickly come unstuck, and you will be the last to know.
The amount of support many teens needs is considerable. The emotional care and the refuge that home should be, can not be underestimated.
Children and teenagers need their parents now more than ever, and I for one want to be there for them every step of the way until they are old enough to charter themselves across. As parents we are anchors, safe harbours, there when they need us - it is the one thing boarding schools prevent, being there to hold your child at the end of a long and hard day. It is something I would never give up.