I fully agree with Mrakobes on this. One thing I haven't seen mentioned aboout this subject though is that there are several things the FNU can do to mitigate the advantage the PL has.
1) First of all is the choice of which countries to disclose. The FNU especially should make sure to disclose neighboring allies if possible, as if the situation Mrakobes mentioned Does happen (PL has a secret ally right next to one of these nations), the other country could lend some support.
2) Choose to disclose allies that have fewer borders the enemy can attack from. That will lessen how much your army is spead out.
3) Cover your borders with sufficient defenses before choosing a specific country to target. So if you have a 3-bordered country, you might end up having a couple LAVs on two of your borders and your main assault force on the third.
4) If you don't have a better country to disclose, and the one you do isn't easily defendable (too many border spots, capital close to a border), consider placing a unit in the capital. That way you'll have a chance to respond to the enemy if he does enter, and hold them off long enough for reinforcements to arrive.
5) Finally, select which units you purchase with care. It would have to be a very special circumstance for me to even consider buying a RL in the disclosure phase as FNU. They are just too expensive to risk having a neighboring PL ally snipe them on the first round. Buy mostly cheaper units as they are more cost effective defenses. And if you do want a heavy hitter, get a decently protected Heavy Bot. Even if the enemy does happen to have enough material to take it out, he/she generally would have to expose some expensive units to do it.
In conclusion, whether the fact the PL always goes first is too big an advantage is still debateable. There is no question this leads to different strategic approaches when playing the two sides, but that doesn't mean that one necessarily has the advantage. The PL is pretty much on the offense to begin with, and the FNU is on the defense. As in war, if the offense is weak or the Defense is too stiff, the PL could be in a lot worse position if one of their attacks fail.
The same principle applies to Chess. A descent White player can sometimes control the game against an equivelant opponent. But a better black player will turn the tables, and use White's strategy against them. For this reason I always choose black when I play chess. I love letting the white player choose the strategy, and then thwart it so they are scrambling to defend themselves. Doesn't always happen, but when it does its fun