I had a similar issue: I was loading the key from the SF2 Big Bundle Upgrade, then the key from SF1 "big bundle" (USMC, UK, NATO). The game would launch, but stick on the splash screen. Once I entered the original SF1 base game key, all was well. Poked around in a few scenarios and campaigns, and everything is running smoothly.
I start 2 weeks vacation on the 14th, looking forward to sinking my teeth into this game.
Also - to the development team, thanks for your hard work, and thanks to Battlefront for the discount!