Both are very good.
Adrien is a playmaker, can play in midfield or as link with the attack (Garmendia-like). He was chosen as Tercera's best player last season, when he played for Baskonia. He also scored many goals for a midfielder (9).
He's having a horrible season, as the whole Bilbao Athletic team yes, but it shows more in him (and Beñat). Why? Because Liñero's tactics consist on constant long balls from defense to attack during the 90 minutes, without passing through midfield, and people who like to play the ball as Adrien or Beñat are totally lost with these tactics. Long balls should be a resource, as any other, but not a constant. We have the best example currently in the first team. So, pure strong defensive midfielders as Eneko or Oinatz enjoy more football in BA than Adrien or Beñat.
Julen is a forward who can play on the wings, is currently playing on the right. Started in Baskonia but was promoted to Bilbao Athletic given the great level he was showing. Caparrós likes him a lot, he's fast, strong and has goal.
Muniain is called to be someone important for Athletic (let's cross the fingers, he is still very young) but I wouldn't be surprised to see him sharing changing room in the first team with his cousins, both have enough football to be Primera players, let's see if the 3 of them develop as expected.