They are flawed for many reasons, here are some:
1. The CT:T ratio, when do you ever win a site but there's 5 CT's alive but you only have 3 and none of the CT's are damaged?
2. Everyone still has their nades
3. Spawn points are completely fucked
4. It's just another game mode where people try to bait each other to the max so the 'retake' never goes down as it would in a real game
To answer in 1 sentence: It doesn't realistically simulate a retake scenario.