What is the disadvantages of simulation?

A disadvantage of simulation in comparison to exact mathematical methods is that simulation cannot naturally be used to find an optimal solution. There are methods which long to optimize the result, but simulation is not inherently an optimization tool. Simulation is often the only means to approach complex systems analysis. Many systems cannot be modeled with mathematical equations. Simulation is then the only way to get information at all.

Another disadvantage is that it can be quite expensive to build a simulation model. First, the process that is to be modeled must be well understood, although a simulation can often help to understand a process better. The most expensive part of creating a simulation model is the collection of data to feed the simulation, and to determine stochastic distributions (e.g. processing times, arrival rates etc.).

Another key point is to ensure the model is valid, i. e. it's behavior mirrors that of the original (physical) system. For systems that don't exist yet, because simulation is used for planning it, this is especially hard. Unsufficient validation and verfication of a simulation model is one of the top reasons for failing simulation projects. The consequence is false results, and this lessens the credibility of the method in general.