"The Cassia is not as safe in larger amounts because of a substance it contains"

Where did you read that? I did some searches, and read that cassia seed pods can be toxic to cattle and horses in large quantities, but saw nothing about human toxicity. In high enough quantiities though, everything is toxic, even water. Cassia seems safe enough in reasonable quantities(perhaps a teaspoon or two or two or even a bit more per day).