The most UNPOPULAR faction in Heroes 5.

The only problem with Humans is that they are difficult to play without suffering losses on neutral territories.

I only have online experience playing as Humans, and I didn't notice anything particularly difficult on Veteran difficulty (at least, I didn't fall too far behind my opponents). The standard strategy is to first crush the weak and slow units (archers shoot single-target attacks at peasants, distracting them, while swordsmen finish them off as they approach; all of this is done either under Haste or Divine Might), and then you can fight the rest, even those with teleportation and resurrection abilities.