Country: England
Feel like this is a myth these days but due to pure bad luck it remains valid. Quite a few arsenal players have gotten stupid reds, there was a segment on sky sports about Arsenal’s poor discipline under Arteta. Because Xhaka is the only one with a history of reds it stands out though.
As for the ‘costing us x points’ again this is one that he’s pretty much gotten rid of, what’s left is Arteta’s arsenal putting defenders in positions where they are likely to mess up. Leno, Gabriel, Bellerin, Holding, Luiz, Mari and Xhaka have all made errors that have cost us goals/points last season.
TL:dr version I don’t think he’s a liability these days, the team has simply gotten worse and everyone is making more mistakes.
It's not a myth, the evidence is in front of us and we've seen it every season since he joined. Especially the errors, did it under Wenger, Emery, Freddie and now Mikel. It's not a coach thing.