I was on pinterest and read a really really really good Steven Universe theory about fusion that made complete sense.

Some fusions will be more stable than others because of the gem they have/are

Like Opal, she is stable and has only 1 set of eyes & 2 sets of arms because Amethyst and Pearl see things the same way but would act differently. 

sugilite is NOT stable because Garnet and Amethyst see things differently AND would act on it differently

Garnet is stable because Ruby and Sapphire see it differently  but would act the same.

Sardonyx would be unstable but Sapphire see's things the same way Pearl would. So tecnichally it is a stable fusion. Ruby and Pearl see it differently so thats why they have 4 eyes. They would probably act differently too.

UNDERSTAND?!?!?