Remember that you get 2 ingots out of a block of ore by macerating/pulverizing it.According to the wiki, it costs 6 million RF to generate a single block of ore. As the yellow filter also produces gold, sulfur and glowstone, assuming that the chances of each are equal, on average you'll spend 24 million RF for a single block of yellorium ore. Does reactor fuel generated from that single block return more than 24 million RF?
It might also be a question of which modpack/configs you are running. Some modpacks boosts the power produced compared to the default mod values.
But I can tell you that it really works very well in FTB Monster.