We are aware of this fact (less borders than other elements), but the border objective is balanced anyway because the borders are really different from the other kind of objectives and so we noticed during the testing phase (hundreds of games) that an equal number of border would result in an automatic victory for the "border" player (if he knows how to play it well).
So don't worry, there's enough borders, but of course you can play differently if you prefer...
That was also my conclusion after my games. You got to know how to play borders and might be a bit trickier, but it feels well balanced. I'm happy to see that it's the conclusion of the creative team as well.