Yuna appears to be unbeatable because her rivals beat themselves. In the fantasy world, it's much closer than that.
Nicely said FlattFan.
Yu-Na is an incredible talent and skater. The thing that makes her unbeatable is the fact that she often executes her content and skates to about 90% of her potential the majority of the time and has skated to basically 100% of her potential twice (Olys 2010, Worlds 2013). Mao and Carolina do not do that. Mao seems to top out at about 60-70% of her ability and Carolina at about 70-80% of her ability. Both are capable of much more than they put out...
If Mao skates her content to its max potential, she will win. Simple math tells us that in the fantasy world of everyone being perfect, Mao would always win based on her superior technical capabilities and her ability to max out the levels on everything else she does. She'd still probably come in 3rd in both PCS and GOE but with her +10 boost in TES it wouldn't matter.
If Carolina skates to her max potential, it would come down to hundredths of a point b/w her and Yu-Na...they are that close across the board if both skate to their potential. Assuming Carolina upped her content, she could actually beat Yu-Na in TES (her ability to do a loop gives her more options in the FS). Given that they are so close in PCS, it would be a very tight race b/w the two of them.
In the fantasy world (where everyone is perfect) Mao is the unbeatable one and Yu-Na and Carolina are the ones jockeying neck-and-neck for silver.
However, we live in reality
In reality Mao frequently under-rotates her jumps and her bigger combos are often more miss than hit. This impacts her GOE and PCS negatively and she finds herself chasing the other two.
In reality Carolina opts for easier content and often makes at least one mistake (usually a major one, like a fall) in nearly every program which hurts her TES and prevents her PCS from maxing out (though she still gets very good numbers).
And in reality, Yu-Na is the one who is capable of skating at or close to her max potential every time out and, as a result, she is usually the winner.