The RC4 stream has biases. This means you have something like "bit X is slightly more likely to be 1 than 0". This isn't "predictability" in a sense that you kn

The RC4 stream has biases. This means you have something like "bit X is slightly more likely to be 1 than 0". This isn't "predictability" in a sense that you know what it'll be, but it's bad enough for certain applications. How much it matters probably depends how exactly you design your RNG, but it seems like playing with fire and is easily avoidable by using a cipher without that property.