The accuracy of SimCraft depends on how many people worked on the simulation for your spec and how many decisions your spec has to make.
Simcraft results are based off of user contributions and theorycrafting from skilled players.
For popular specs, especially ones used at high levels of raiding, a lot of people go over rotation possibilities, trinket and glyph combinations, and stat priorities, experimenting until they find something that generally works well in a lot of situations. If you play a not-so-popular spec, then you don't have the benefit of so many people tuning the simulation action lists, and you often end up with an underperforming simulation - in which case, it's easy to do more than "max dps"!
In addition, sometimes there are just too many factors for SimCraft to handle. Factoring in how to snapshot while using Alter Time or dealing with variable movement in a fight can sometimes result in wildly different predictions than reality. However, relative predictions (like how much will a piece of gear increase your dps) should be more reliable.
Since the answer to "how accurate is my simulated max DPS" will vary so much by spec, you're probably better off consulting class guides, looking at parses from people around your gear level, or factoring in SimCraft results with other simulations that might make different assumptions about the fight. It's hard to get an exact "Max DPS" number, but with a bit of work you can get a general range for where you should expect your DPS to be.
If you want more information on why SimCraft isn't perfect, but can still be useful complement anyway, I'd recommend reading:
