Sporting Kansas City is a hard team to go up against in their stadium. An early red card didn't stop them from getting the result they needed against FC Dallas to advance to the semifinals of the US Open Cup. FC Dallas is on a home six-game unbeaten run, but they are struggling away from home.
Offensive struggles (they definitely aren't gone yet)
FC Dallas had about 85 minutes to try to break down Kansas City with a man advantage. That was with Dallas' full attacking arsenal except Kellyn Acosta, and was against a Kansas defense missing both Graham Zusi and Matt Besler, who were with the USMNT. They also obviously didn't have Seth Sinovic, the player who was sent off. These offensive struggles are the biggest worry for FCD going forward.
FC Dallas was too predictable in their attacking. The only plays that caused any problems involved Diaz playing the ball through to Barrios, and if the runners in the box were well marked (which they were), not much came of those opportunities.
Inability to break down the bunker
After Sinovic got his red card, Sporting KC bunkered down and defended, with the occasional counterattack. This seems to be the best way to go up against FC Dallas in general. The crossing ability isn't there, and more importantly, Dallas doesn't have a strong hold-up forward who is good with the ball in the final third and can smash in headers or loose balls in the box.
Also, Tesho Akindele was disappointing in this game. He can be good in space, but he wasn't affective against the tactic used by Kansas.