How so?

Screen reader's probably not going to catch a transient element unless you just happen to stumble across it within that narrow window. Slow reader for whatever reason? Hope you don't take too long, or hope that toast wasn't actually important/actionable for you.

The spec has an answer for the "transient element" issue: https://developer.mozilla.org/en-US/docs/Web/Accessibility/A.... Of course, this doesn't eliminate the possibility of bad UX.