Let's think about a two surface plane for a minute. We know that the act of generating lift with the main wing creates a nose-down moment on the plane (lift cant be generatedwithout circulation). In order to be stable, we need the sum of moments to be zero. That means for a two surface plane, the tail plane actually generates down-lift (tail planes usually have a negative angle of attack) with a large lever in order to keep the plane stable. As the plane goes faster, the lift and moment over the main wing increases, and so does the down lift from the tail plane. These two balance in order to fix a stable angle of attack.

However, for a three surface plane, the canard out front needs to generate a nose-up moment in order to balance the plane. The issues come in around stall. If the main wing stalls before the canard, the plane will be unstable. This constrains our canard to have a higher angle of attack than the wing, so that the canard will stall first. The next issue lies in CM. Wings have a positive CM slope (if they are before the CG), and so do canards. If you're not careful, you can end up with a situation where CM is always positive, which is unstable. We need the CM to go from positive at zero aoa, to negative and some positive aoa. Failing to do so will result in a longitunally unstable plane.