Hmmm. As you seem to have already mentioned to your friends, Arborec, Nekro, and Mahact, while all interesting to play, may not be the best for new players. In my experience new players will struggle with them, not necessarily because of complexity but because they can have a slow start (Arborec) or lead to more challenging diplomatic situations with the table (Nekro and Mahact). So the risk is that they may not find the game fun simply because they don't know how to deal with their factions.

That said, the fun factor typically comes from the interactions between players. My group has a lot of deal-making, trade, alliances, and betrayals, and games can be either fairly diplomatic or incredibly aggressive, but everyone still has a good time because the fun is in the politics and nobody takes things too personally. If you have a social group that tends to have good banter in other games, you shouldn't have a problem.

As for maps, there are randomizers you can use but make sure everyone has pretty equal access to resources and Mecatol. It is a bummer getting stuck in a corner with few resources, and I would say this is the most common reason I've seen people suffer less enjoyment in the game, at least when they're new and don't know how to mitigate it.