The Final Obstruction

There is but one limit to a Wraith's travel in the Deadworld - or, more properly, out of it: the boundary between it and the Spirit World, otherwise known as The Barrier.
 It is said that once, long, long ago, the Spirit World was as much a part of the world as anything else. People could go from the realms of the living into the realms of the dead, provided they were willing to make a long and arduous journey to do so. And it was not unheard of for the dead to make the return trip, though it was rarely for long - death is, after all, a very jealous owner.
All that changed with the Sundering, though, and now the Spirit World is off-limits to Wraiths. Between the world shared by the living and the dead and the Spirit World is an invisible obstruction known as The Barrier, which forms the single boundary for all the dead. It is all around them, like a blanket they can never slip out of, but could rise up and strangle them at any time with a Storm, a Reaper or one of The Damned.
Of course, the Barrier is not completely impenetrable. Some inhabitants of the Deadworld traverse it all the time. And these entities have been known to take Wraiths and Ghosts back with them, willingly or forcibly, for their own, unguessable purposes. But while they are able to make the journey with seeming ease, no Wraith who has tried to follow them has ever been seen again.

"No Wraith?"

Oh, I went there, once, I did, I went through the Barrier, Gears came back, I went there and Gears came back, Spiderman came and took me through, metal legs and steel teeth and all, reaped me, he did, raped me, reaped me, took me to the place where they all come from, Spidermen, heard the eggbeaters whirling in the sky, saw the black smile of a sky up above, dark angels pissing on us between the teeth, stolen moon sky, broken teeth stars, hungry clouds thinking heavy thoughts and flattening the world underneath, Spidermen took me, made me not me, Gears came back, Dark Mother Machine took me in her coils and corners, took me back inside her, operated on me in her love surgery, fucked me and made me the machine I am today, what he is today, I am, he is, I was, he is, I went through it, Gears came back, I went through it, Gears came back

- "Gears," First Prophet of the Mother Machine, Believer of Tulsa.

The Great Unknown

Through the Barrier lies the Underworld. This is what The Order proclaims, what the Believers preach, and what the Ferrymen will say little or nothing about. That is not in dispute, except by those with a gift for sad denial.
What is in dispute is the exact nature of it.
The Order says that the Underworld is the realm of Charun, His fellow Gods and Goddesses, and those worthy few Wraiths who serve them faithfully. They speak of a long, dark river that leads to the gates of the great city of the dead. And they speak of the rest, respite and surcease they shall all find there, when the cares of their lives are lovingly taken from them, and they can at last have the peace of mind they crave. One day, the Barrier will fall, and the shores of the river will lap at their feet, ready to carry them home.
The Believers generally agree with this idea, but they differ in what face the city of the dead, and the Gods to tend it, wear. Many Believers base their views on monotheist religions, and expect Paradise to be akin to Heaven, as interpreted by Christianity, Judaism or Islam. They expect to find God on His Throne, with the angels in tow, and think the Damned have been sent by the Devil to destroy those weak in faith. Other Believers follow older, Pagan creeds that have a different God - or Gods - on that throne, and a different way of spending eternity.
As for the Ferrymen, they won't say what awaits, other than to call it Paradise and encourage Wraiths to resolve their Anchors so as to travel there with them. Their silence about what awaits is never explained, except to say that blind faith is the greatest weapon against the Shadow.
That and patience - silent and sure.