Well it's funny you say that you dislike characters whom seem to have the author go easy on them and have them be successful no matter what. If that's your take on it then you shouldn't like Soo-won at all. He's literally being handed everything to him on a silver platter, most of with doesn't even require him to get involved or lift a finger. And so far, he hasn't failed once.
It seems like you forget that during the battle between the fire tribe and Soo-won, Yona was there to try and break things up. And she failed very early on trying to achieve that. There is your failure for the MC. Not everything that Yona does must result in failure. Otherwise why bother reading the manga if all you want is for the main character to fail over and over, and expect problems, that would require outside forces to help, to resolve themselves.
I don’t if you have noticed, but I was compering two main characters, not a main character to a supporting character with a considerable less pagetime. As far as I remember this manga is called Yona of the Dawn, not Su-won of the Dawn, so showing his struggles and failures is not of equal importance and since he is not the main character, it can be more easily assumed that his struggles happen mostly off-page. Although I would certainly not mind if Kusanagi showed, for example, him struggling or failing to achieve something in the past.
As for Su-won being handed-down everything on a silver plate, we were shown that he was learning in the past, so it is easy to assume that he is reaping the benefits for his past effort now. The same goes for Hak. Do you see Hak struggle with mastering his fighting skills in the present?
Besides, even if Kusanagi showed Su-won fail, which I would not mind at all if it is done well, it would only show that he is a human being that can make mistakes and that’s it. It will not magically make Yona a better ruler material, because it would be as ridicules as showing Hak make a mistake in a fight and then claiming that Yona, who didn’t do the mistake, is a better warrior than him.
I'm probably going to be stepping on a lot of toes here.
In my opinion, I think both Yona and Soo-won suffer from too many winning streaks so far in the story. Both characters have one too many perfections in either their character or actions for me to find appealing.
Yona, aside from her supernatural importance, has a personality that is inspiring and so sweet, it's cloying. Seriously, I can't find any instances where characters didn't find her charismatic or attractive. I'd like if there was at least one tick in her personality that wasn't so glamorous. For example Kagome from Inuyasha had a bit of a short fuse, and Miaka from Fushigi Yugi had a gluttony problem. What does Yona have? Vanity? I only saw that in the first chapter. Now it appears she enjoys traveling and can get more rough and dirty than Kija. Naivety? Naivety is something everyone has at some point in their life and it doesn't count as a personality defect. It's only a personality defect when one can't let go of their ignorance or naivety. Clearly Yona is not in this category. Stubbornness? Usually Yona's instances of stubbornness happen in a self-sacrificial manner. For example when her companions urge her to rest, but she stubbornly refuses because she wants to help someone or strive for progress. You see. She is sweet, adorable, caring, beautiful, fierce (in a majestic, heroic way), strong-minded, inspiring, self-sacrificial, I could go on...
I don't hate Yona. In fact sometimes I find her sweet and cute in a good heart-warming way, but other times her sweetness becomes a bit too overwhelming.
Soo-won's character has a different sort of problem than Yona, but no less aggravating in his perfections. As
@BlackVice pointed out, Soo-won's genius and success as the new ruler of Kouka is so ridiculous in its greatness, it makes all other characters look like meandering fools in his presence. Though unlike Yona's shining grace, Soo-won has a black stain on his character for murdering Il and betraying his closest friends. You'd think that'd help my reservations with him, but it doesn't. In fact this dark task he did almost seems justified because the world needs a genius leader like him. The way it's been portrayed so far in the manga, it almost looks heroic and self-sacrificial in a twisted sort of way. You are almost supposed to pity him for making the necessary move to save the country. That sacrifice causes him to lose his friends' trust and make him morose because of it. You are supposed to admire him for burying his emotions and human attachments -a very hard thing to do- in order to become the impassive, perfect King that will save Kouka. And so far his winning streak as a leader reflects that. The generals and enemies underestimate him initially, then quell at his magnificent strategy and character. Rinse and repeat.
But Soo-won is a fascinatingly complex character. Like many other people, I can't help but try to puzzle him out. Kusanagi hides his thoughts more than other characters to add to the mystery of Soo-won and to keep you guessing on his next plans. However, what makes Soo-won unique is not because of his personality, nor his actions as a leader. What makes Soo-won unique is his position in the story from a literary perspective. Soo-won starts off as the supposed friend-betraying evil villain/antagonist that takes the throne and will need to be righteously taken down by our hero. It's what you typically see in so many other stories and is what you are initially lead to believe how the story will take off. But that's not what happens. What throws the reader for a loop is finding out later that Soo-won is not your typical antagonist, if even an antagonist at all. I do applaud Kusanagi for this masterful stroke in the manga, because this deconstruction of the typical hero vs. villain format is what really makes Akatsuki no Yona stand out as a top tier manga in my opinion. And one of the reasons why I love it.
However, I do not let this blind me to what I mentioned earlier about Soo-won. If he were the main protagonist of the story, those perfections would be far more obvious and I think would have made him less popular of a character. He is not unique because of his characteristics, he is unique because of his place in the story. As
@Aylinn pointed out.
I know the manga is not over yet, and either Yona or Soo-won are bound to face failure. What I'd like is if the failure was due to a personal mistake. That would take away from their perfections and make more appealing characters in my opinion. I respect Yona and Soo-won as plot-engines but await the day I see more to their characters I like. So thus I am open to changing my opinion of both of them, and am waiting in the wings in a sense.