Most flight simulators don't actually simulate every aspect of the aircraft, and may only partially simulate some aspects. At first this might seem like a bad thing, but when you think about it there are many aspects of an aircraft which are irrelevant to training. Does your simulator need to have the heat and air conditioning simulated if you're operating it in a room which has it's own environmental controls? Of course not. At the higher level of simulation (eg. in the US FTD Level 6 & 7, as well as FFS) failure modes must be replicated but the environmental system in use does not have to replicate one-to-one the environmental system in the aircraft.
It's hard to generalize what is and isn't simulated on different simulators but a good general rule of thumb is that more expensive simulators simulate more things and in more detail. A basic simulator may not be able to simulate certain failures such as a tire blow out during landing. It might be able to simulate the landing gear not coming down at all, but a tire failure is a different and difficult thing to simulate.
A common example of something that often isn't simulated is physical breakers. In many simulators when the instructor wishes to simulate a failure in a particular system they tell the simulator that system is not operational and that's the extent of it. It remains inoperative until they press a button to re-activate it. In a more sophisticated simulator the instructor can tell the simulator to blow a breaker which will cause it to trip1 and when the student resets it the circuit will become active again. This provides a different training situation than a mechanical failure in the same system.
It's important to determine how you plan to use your simulator, what specific situations you want to be able to simulate and to what degree you require them to be simulated. That will allow you to determine just how sophisticated of a simulator you need. As well as allow you to evaluate various offerings based on how well they will fill meet your training requirements.
In higher level simulators there is generally a regulatory requirement that if a simulated breaker is tripped any systems which are powered on that circuit must act as they would in the real aircraft in the event of a power loss. ↩