I haven't played the expansion to say anything meaningful about balance, but I think I know about learning curve from my very first 4 games:
Free MrWeasely versus Shadow Alan Bright: Alan's 10th+ game of expansion, my 1st: Free victory. Free MrsWeasely versus Shadow MrWeasely: MrsWeasely's 1st full game of WotR is an expansion game. Free (barely) beats the 50x more experienced Shadow by rolling lots of swords. Free MrWeasely versus playtester Seven Sprits: Free victory. Free MrWeasely versus MrsWeasely: Free victory, with a turn or two to spare.
So I have yet to see a SP victory, even with heavily jet-lagged and/or green free players.
I've seen Shadow Military Victories, four out of roughly fifteen plays... however, I've yet to see a Corruption Victory where FP didn't make any serious blunders (such as separating 10 points worth of companions). It could be that the Shadow is simply harder to play with the expansion, and once we get the hang of it the statistics start correcting themselves... however, I'm having especially hard time corrupting the FSP.
Wow... my first... 8 expansion games, I think? Were all Shadow victories. All were fairly close, and we could see places where lucky rolls or cards tilted it, so I never felt like the FP were getting shafted by the xpac, but for a while I felt that the xpac was definitely favoring the Shadow. Interesting to see an alternate view.
But we were really excited when we had our first FP victory in ToTTA.
i feel the FP have a strong edge in the expansion -- they got galadriel, 2X better ents, 1.5 extra elven rings, and siege engines that help them a bit more than the bad guys. The bad guys get corsairs (fun but neutral due to FP card), dunlendings (useful, but imperfect), and the balrog, while effectively losing the voice of saruman.