mix SAS and SATA HDDs in RAID 0

Specs for some RAID controllers (for instance LSI 9261-8i) say that it's possible to mix SAS and SATA disks in the same RAID array but such practice is discouraged (but without explanations why). There was mention here on SF that some admins use such mix to minimize cost(on software RAID, my case is hardware RAID). What will be consequences or drawbacks of such mix?

but such practice is discouraged (but without explanations why)

Most RAID controllers run better when HDDs are close to each other in speed. Different models/interfaces break that. Many controllers don't even permit arrays with mixed drives (by grade or interface).

Be prepared that any enterprise-grade drives don't speed up an array made up of nearline or consumer-grade disks. In other words, your array performance is likely determined by the slowest drive.