Multi Factor Authentication is obviously a lifesaver for passwords, so things that can easily leak (peeking, guessing, stealing, ...). A second/third/... factor of another kind considerably reduces the risk.
This is less obvious for hardware tokens/cards (the "something you have") - they can get lost or stolen, but you usually mean it. Still, having an extra factor helps.
For biometric, the risk is even lower because you either need to correctly bring the factor to the sensor (grabbing an eye or cutting a finger would not usually work), or to kidnap someone for their biometrics and then there is probably much more at stake. A second factor is only, IMO, a nice to have.
What I mean in general is that the nature of the unique factor drives the "absolute need" vs the "nice to have" of consecutive factors.
The above is a reflection of mine and I am not expecting a discussion here (because it is not the nature of this site and the question will be closed as opinion-based), but a reference to either a standard or a good write-up on the topic.
I could not find anything besides the general "you need more factors" (this includes NIST).