I think you need to kill all the Hordes before completing the last mission to get the hidden ending
No just all the Nero missions and the Nero sites and you can complete them after the big ending if you miss any.
The only thing that annoyed me were the flashbacks before the world went to hell where you had to just press walk arrow to get through the scene, you couldnt skip them.
Its not like Far Cry at all. Its more watered down.
It is fun though when you start to experiment.
eg before taking on a camp at night drive round and gather up some zombies then drive slowly to the camp, rev up drive straight through it and let the zombies kill the marauders.
Find a cliff or a steep drop, run around and gather some zombies, run straight at the cliff then hide nearby and watch them go straight off the cliff. Or find a cliff chuck an attract bomb at the edge, zombies go there get blown up and watch them fly over the cliff.
you also get other random stuff where you are about to be set upon by a few zombies suddenly a deer appears and they forget you and go chase it and you just stand there looking thinking WTH!?! Loved those moments.
Oh and the ? annoy me sometimes too where you go to find what is happening and it ends up being a few deer or a couple of zombies fighting a bear. Sometimes its good and its people to rescue or even an ambush.