If your meeting takes too long, then the efficiency of each team could drop, raising the costs associated with holding the meeting. You also don’t want to unduly bore your employees. You wouldn’t be the cool boss if you did that.
The other very important aspect of billing a flat fee project is that you must hold your client to their scope of work! When someone is buying something from you for a set price, the things you are agreeing to do for them must also be set.
A complexity represents a nonlinear growth in overall development & QA time, which can crush your project!