How to perform realistic resource leveling.Is resource loading in Primavera P6 Schedules dependable?

Try to consider the following:

1. Activity-logics; Activity type

2. Global Calendar

3. Resource dependencies

4. Resource Calendars vs. Activity Calendars

and most importantly;

5. Critical Path / Activities

Yes, it is really a nightmare, your resource leveling is as good as your resource loading. 

I supposed you will only level resources in the critical path, but remember that the critical path is also a changeable thing. So everytime your critical path changes, you will also need to level your resources. 

hope this is clear