I read a lot about how to give praise to children because so many child prodigies grow up to be disappointed, depressed low achievers. A lot of that sadness is a result of people telling the kid over and over again how great they are. It robs the child of incentive to do the hard work required for anything substantial, but also, it gives the kid no sense of control over their lives — they did not earn that greatness, they were born with it. So they feel that they have no ability to earn greatness, they just have to wait for it.
So for cello, I focus a lot on how hard my son works, and I say something specific like, "I like how carefully you're working on that scale." I try to focus on the hard work rather than easy comparisons to others. Also, I tell myself that this will help us both make sense of the world if he chooses not to do cello. We are not aiming for a world-class cellist but a world-class person and hard work and diligence in cello is a step to create that person.
So I pay special attention when I read about how to compliment employees on work well done. Employees do better work if you compliment them in a way that's customized to their needs. Different people have different intrinsic motivators, and they care about different things. For example it doesn't work to compliment someone on their social skills if what they really care about is power.
It makes sense that kids would need customized compliments just like they need customized learning. However kids get told what to learn and how to learn it, which makes customized encouragement seem hallow.
I also realized that as parents, we appreciate our kids more if we compliment them based on who they are and not based on what we think people should want to be good at. Self-directed homeschooling allows kids to do what matters to them which in turn allows parents to compliment kids on the process of doing the work that matters to the kids.