How are Xayah and Rakan balanced?

They can be really good together or really bad apart.

Or they could be really good together and ok apart (which is how I think they are).

Or they could be ok together and bad apart.

So how are they balanced if they're made to be better together? Are they made to be unbalanced? I don't understand how they could possibly be ok apart and ok together if they have such good boosts together.