I was reading through the Material Design guidelines and noticed this restriction on the snackbar usage: "Don’t place icons in snackbars."
Why not? What would be the rationale for that?
I think the answer is about user interaction with Snackbars. When a Snackbar (or toast!) appears on screen, I just scan it quickly to check everything went well. Not reading the content of the Snackbar. Using icons needs more attention to understand the message. As you can see "Alert" is recommended on Google Material guideline.