This answer might surprise a lot of people but I found the legendary birds in XY the most infuriating roamers. They technically become stationary eventually I know, but I thought they were worse to catch than the legendary beasts or the Lati twins, Mesprit or other roamers because in the previous cases at least the legendaries become fair game as soon as you see them. You can throw your master ball or try to trap them with mean look or shadow tag. But with these legendaries you encounter them in the wild but can't do trubbish about it. They run away automatically and you are not even prompted to catch them.
This is not like a one time thing. You have to run into them over and over again like a dozen times. Wtf? Why?
This is not say like Suicune in the Johto games where she interacts with your trainer sprite and runs away a couple of times at a set location as part of the story. Your encounter with the legendary bird is completely random.
I have never caught one of the birds in XY. Its just way too much time investment. I'm willing to ride up and down the route to trigger 1 encounter, but not 10. I just don't really see the point of this. Its so tedious, and unlike with Eusine's character in Crystal/ HGSS there isn't much explanation for why this is happening.
Q: What is worse than looking for a roaming legendary?
A: Having it flee from you.
Q: What is worse than having it flee from you?
A: Having it flee from you 9 different times.
Q: What is worse than having a roaming legendary flee from you 9 times?
A: Having it flee when you had a friggin master ball right there in your bag each time.
You don't know when your going to encounter this Pokemon again, it can change routes. You have to have this same fruitless encounter over and over again, with no real explanation. No, it just wasn't even worth it. I have caught roamers before but not these.
I was at the end of Pokemon Y and had done everything beat the E4, conquered the battle chateau, Maison and institute, looker's side quest, caught all the other legendaries, and still had seen Moltres only twice. No, just no...