First of all, Universe has certain Conservation Laws that are ALWAYS obeyed: Mass|Energy, Linear Momentum, Angular Momentum, Charge, Lepton number, Baryon number. Any proposed decay process that violates one or more of these is IMPOSSIBLE and never occur.

On the other hand, any decay process that obeys ALL the conservation laws MUST OCCUR. That is, if you consider a nucleus, then rearrange its components into a smaller nucleus along with assorted other particles in such a way as the Conservation Laws are obeyed, then you have written down a REAL decay mode of the original nucleus. ALWAYS.