The speed of causality is a fundamental speed limit. It is also the speed of light, the speed of gravity waves and so the limit on the speed of communication or travel. Exceeding the speed of causality would allow the creation of contradictions which as far as is known, are not possible and would subvert physics.
I've heard many people much more knowledgeable than me state that FTL phenomena/communication would violate causality, but I've never understood why that is or why that violation should be fundamentally impossible. Simultaneity itself is demonstrably relative and it doesn't seem, to my layman's brain, that causality couldn't also be in some form relative.