Adama was also in the TOTY and among the best players in Segunda.
Some players start brightly, but then fade away due to huge holes in their game, others take time and improve with eventually turning out to be the better player.
Not saying Arnaiz will be a better player since I know little of him, but being that great at 18/19 is pretty much irrelevant when it comes to being great at the top level because you have to keep improving which Adama failed to do and Deulofeu so far has failed to do as well, although he's showing moments recently where it looks like he can still make it.