Number of Nav facilities are limited
Enroute alternates are also limited
Magnetic dip is maximum hence the compass will be inaccurate
Position of Magnetic Pole changes (Variation Rate is high)
Compass can't read anything in the 6 microtesla zone (horizontal component too weak)
Navigation sets required on board for Long Range = 3 (redundancy, use 2 and isolates the unserviceable one)
If you have only 2 and one is unserviceable, to identify the unserviceable one you cross-check with a fix, you contact other aircraft and you compare with the one that you have.
Minimum Time Route = the route you take to reach a particular destination under minimum amount of time while taking into account the winds and etc