Be patient. At first I thought it was broken too, but Rivera loaded after I let it sit on the white screen for 25 or so seconds. My supercard did this with a few other games (sometimes it was a black screen), but as long as the Game Boy Advance logo wasn't scrambled the game would run.
Also, after it booted up slow the first time, I never had the white screen issue the next time i ran the game.
I haven't had any problems or slowdown with the gba games I have played. ( just dont use compression).'
Edit: Riveria does work for the supercard (as do almost all gba games) dunno what above poster is talking about.