Simulations are only as accurate as the data entered into its variables and the calculations it uses to solve.
Simulation software uses proven, higher-level formulas for its calculations, which are normally based off of scholarly methodology. These methods are usually published within the software documentation. Simulation software developers scrutinize and validate the calculations to achieve reliable simulation results and often work with real-world users to identify and resolve potential inaccuracies.
Although simulation does provide insightful data for your designs, it is highly advisable that you use your expertise and industry knowledge to judge all simulations’ trustworthiness. If your design is extremely unique or a brand new way of approaching a problem, you may consider the simulation as a starting point leading into a prototype that proves the design.