Mix & Match: Favored Enemy/Environment

Would it be unbalancing to house rule that you can mix and match some one-for-one tradeable ACFs?

For example, Unearthed Arcana has a Ranger variant that lets you pick Favored Environments instead of Favored Enemies. Would there be anything wrong with allowing someone to pick FE (Hills) at 1st level, then FE (Undead) at 5th level, as long as you don't allow the bonuses to stack when fighting undead in the hills?

If the bonuses don't stack, then no, there are no balancing problems that I can see.

Even if they did stack, they might be underpowered, so no there's no problem at all.