I think an important point to focus on is reasons for group cohesion. Rightly or wrongly, evil groups are more likely to fall apart into inter-party bickering, in my experience. If you're running a traditional game where the PCs are goodies and out to save the world, you can usually rely on them just generally being nice to each other. I think it's important in an evil campaign to have them all beholden to more powerful entities/factions (who demand they work constructively with the other PCs), ensure that life without friends is a miserable existence and thus create selfish motivations for group cohesion or do something else.
There may not be any real reason an evil group is less likely to last than a good group, but it's nonetheless a problem worth building in to the PC backstories, in my view.
If any of my players want to make an evil PC I require them to have a reason to stay loyal to the party. Whether that's greed, love, loneliness, orders from superiors, desperation, family, debt, or whatever is their choice, but they must be loyal. Its worked pretty well so far and I don't think it would be hard to apply this to an entire party. Evil people can have perfectly normal relations with the people they are close to, its just that this group is typically very small and very specific.