You get great gains with minimal sides from deca. Winny acts as an anti-progesterone as well as giving you decent gains and boosts stamina. Although it may be a popular stack, it is not the "perfect" stack. When doing cycles you should have a mixture of androgens and anabolics. Winny and deca are both anabolics. Now a winny/deca/sust cycle would be perfect. It would help cut fat and build big, strong muscles.
Winny is a Class II AS making it synergistic with an aggressive AR binding AS like Deca. It helps block progesterone, hardens gains, alleviates the bloat----and Deca helps with the joint pain often associated with Winny use. Perfect fit IMO, if you throw in Test.
Winny stops gyno from Deca, and (correct me if I am wrong) Winny is not particularly joint-friendly, whereas Deca is. So they somewhat cancel each others sides out.