Single pivot bikes suffer from brake jack, and depenending on pivot location, pedaling forces can jack(raise), or squat(lower) the suspension under load.

There are ways to get around these problems (floating brake arm, chain torque eliminator), but it still cant make up for the fact that its still single pivot.

Suspension designs such as FSR have little or no brake jack, due to the way the "seat stay" link moves throughout the travel. It acts like a floating brake. It is also very neutral from pedaling forces too.