I think it can be said very flippantly, but we do use it within the immediate family all the time, particularly to each other and without prompting our children often finish phone calls, Whatsapps to us with it, I'm glad they do, I perceive it is said with meaning, albeit a bit of a generational thing, and of course we reciprocate like for like. I don't think you can tell a loved one that too often, it's a reaffirmation thing.
I felt the need to actually express that to my late mother in a "love you lots, I couldn't have asked for a better mum" sort of way knowing as she aged our time together would inevitably be limited. I know she didn't doubt my love for her, or hers for me, I just think possibly that generation weren't so expressive in all of that. There is a sort of catharsis when you lose someone to know that you've told them the most important thing of all.
I tend to sign off to my friends, as they do with me with a couple of xxxs.