More of a "Is she being unreasonable" than an unreasonable one.
A close friend of mine insists a child that isn't particularly nice is easier to look after if they have a saving grace of being good looking. If a child of equal naughtiness isn't good looking, she insists it's harder to look after them in terms of if being emotionally draining to a point of no return.
Of course, she says, if a somewhat ugly child is a little darling then that's fine and they're lovely in their own sort of way, their behaviour even makes you believe somewhat that they have their own unique look.
She has been a Nanny in her time but is also a mother and said she felt the same way about her 3 children. She admits they aren't as equally as beautiful as the other but prefers to concentrate on their good parts and says her "least pleasing to the eye" DS has the most academic potential but also observed that teachers seem to like her other sons more.
I have to admit that I do understand where she's coming from. I have a 12 year age gap between me and my sisters. Both were equally as naughty and badly behaved but I preferred to look after one over the other because one was very pretty and I found it easier to forgive her (no harsh judgment please, I was only 15 at the time)
Can't comment as an adult because my own DC is stunning and of course I'm completely biased 
What do you think?
Disclaimer: DFriend is very unusual in a sense that she doesn't hold fire when she thinks something and knows it won't cause offence to you personally.