Who told you that?
The main concern is if you have some problematic SAS expander that causes problems. Some of the first generation SAS1 expanders were known for being pretty crappy anytime anything unexpected happened. This includes errors teased out by noncompliant SATA implementations.
Generally speaking, anything manufactured within the last ten years for SAS2 or later *should* be fine. It's still a good idea to avoid mixing them unnecessarily, I suppose.