If you can pick factions on ME that don't have the same climate preference, it's nice since you're complementary. Just try to avoid having as an ally someone that is naturally hated by the factions you're supposed to be friendly early on (like for example doing one of the DE LL with Kathep), as this will make your start very difficult, but that's about it. on the ME campaign, you'll have far more freedom. If you're playing the vortex campaign, you can only do co-op with the same race afaik, choosing another race will be a head to head. So it's not a problem when you combine two of those 4, but don't combine one of those with something else. This results of having factions that would usually be favourable to you hating you for being allied with them, making it almost impossible to ally with any AI and being easily cornered by having too many AI declaring war on you. Be warry about any other commbination with norsca/wintertooth/chaos/beatmen as pretty much any other faction hates those four. Norsca or wintertooth and chaos or beastmen, yes.
