I think the issue around him is based on whether people expected him to be a number 10 (in the role of playmaker) or an attacking midfielder, and there is a big difference in these roles. Obviously a lot of swansea's play, particularly last year, went through him. However in the weeks and days prior to his signing I got the impression from this forum and blues in general that they were hoping we were getting a player who was capable of running matches single handedly, dictating our tempo and creating the majority of his chances from open play. That's just not the player Sigurdsson is.
A point of comparison I would make is at Man United and the difference between Mata and Mkhitaryan. They're both attacking midfielders, but they have very different skill sets, with Sigurdsson definitely being closer to the Mkhitarayn mold.
I'm sure someone who disagrees with me could throw up a clip of him playing a lovely through ball, dinked pass etc . I don't dispute he has that in his locker. And I know there are the stats putting him as one of the best for assists in the the league these last few seasons. But it is irrefutable that the majority of his goals/assists come from set pieces. I'm sure others have seen the stats that prove last year he created more chances from set pieces than any other player. In contrast, he was also way down the list for chances created from open play. Now he's in a better team (hopefully) those open play stats should catch up. But given our debacle in the transfer window in regards to our pursuit of a striker/target man, we've negated an incredibly large part of his game.
Furthermore our midfield to attack transitions are so poor, we're hindering him even more. I know our midfield personnel are probably the strongest area of our team on paper, but there's an issue with them going forward. Klaassen has the ability to make good forward runs, but I would love us to have a deep lying playmaker in the team, someone capable of starting attacks from the midfield area but also capable of pushing forward themslves and grabbing goals. Especially in games against the dross of the league. Can you imagine how much crisper and fluid we'd look with a 26 year old Mikel Arteta in our midfield? It would help people like Sigurddson no end.