Hum, front line attack alters the math of the combat a bit, though not really in a specifically unbalanced way.
It makes fortification less important; unless fortification would be reducing the hits you take by a factor of 3 or more, there's a certain advantage in it, since in the grand picture doing 3x as much damage and taking 3x as much damage results in the same losses, just quicker. If a fight were one formation vs one formation, you could think of front line attack as giving the unit a temporary fortification 3*(1/hit mod) and making the battle go quicker; a formation of static units on attack (hit modifier of 1) vs a formation of static units with fortification 3 would be inflicting the same number of hits on each other.
However, this is only true if all or most of your army is on front line attack, because those targets are weighted more heavily. If you have one formation on front line attack and the rest remaining fortified, then more attacks are going to hit the more vulnerable attacking formation and fewer the fortified positions, which is heavily disadvantageous. So with a few specific exceptions you're either going to want most of your units on front line attack or none of them.
Speaking of exceptions, there's the light vehicles. Light vehicles have a to hit modified of .4, which means on front line attack they'd be on equal footing with a formation on fortification 7.5. This means they'd make excellent candidates for front line attack, and should almost always be used this way (even if none of the rest of the army is). Normal vehicles will also benefit from being on front line attack unless well fortified on a world with a fortification bonus (equivalent of Fortification 5).
So, takeaways:
Setting your entire frontline on attack is basically always a benefit unless you have a quite high fortification level (3+ for Static, 5+ for infantry or tanks, 7.5 for light vehicles), assuming you have the supplies
Light vehicles will benefit immensely from being set on attack
It's overall a major nerf to the defensive side; they're basically robbed of most or all of the their fortification bonus if the enemy can set their army on front line attack, but can't use it themselves without losing the bonus entirely
In all honesty, if it were me designing the game I'd probably drop it; defenders need an advantage since they're going to frequently be outnumbered/outgunned. Actually, I'd probably just limit the field positions to front line and rear echelon; I feel a game as complex as Aurora doesn't need a lot of ground combat complexity as well. However, that's just my opinion, and it's not one I have strong feelings about; I don't feel anywhere near as strongly about it as I did about the preferential targeting, for instance.