Units from the non-aligned forces are always considered to be suspicious allies – one can never entirely rely upon them. That's not to say either party necessarily has any reason not to trust the other – they just can't quite bring themselves to do so. This is true even in an alliance between two such armies – it's quite easy to imagine two Tomb Kings uniting to fight against an invader, with each never quite taking their eye off of the other even in the midst of battle.
Suspicious Allies work exactly like, except that they:
Are not counted as 'friendly' units from the point of view of targeting spells or other special rules.