They can be very accurate if you set the simulation parameters to match your laser, and you don’t exceed the processing power of your controller - the simulation doesn’t know if you’re using an 8-bit controller that can’t keep up with what you’re sending it.
Go to Edit > Device Settings, then click on the ‘Additional Settings’ tab:
Click ‘Read from Controller’ to match the simulation parameters to those in your controller. If you have an 8-bit GRBL controller (your profile says X-Carve) be aware that the amount of GCode those can process is limited to about 250 instructions per second, and photo engraving can easily exceed that. If you see the laser head slow down when you get to shaded bits, that’s what’s happening, and there’s not an easy way to account for that in the sim.