He was good for Reading, 'meh' for Hoffenheim, brilliant for Swansea, 'meh' for Spurs, brilliant for Swansea, now 'meh' for Everton.
Problem with Gylfi is that he's a bit of a luxury player that only looks good when he's the 'star', kind of like Zaha really. To elaborate on that, he only seems to be good when he is allowed a free role, with everyone wanting to pass to him. He doesn't work hard enough to get the ball, and isn't good enough at finding a man (and probably why he's only ever looked good when he's not expected to feed other players with creativity).
In short, he's not a top-8 kind of player. He'd suit a team like Brighton, Bournemouth or Huddersfield, who tend to want to play good football, but would happily let the team revolve around him.