I tend to run mallet dh pedals though like flats in winter (mainly as they force me to ride properly and unlearn the bad habits I’ve picked up riding clipless)
So my opinions disguised as fact:
Mallet 1s are useless, the mechanism sits to far proud of the body meaning your shoes slide over them. No retention pins doesn’t help. The body is too narrow to use wide shoes (5tens in an 11) so you rib on the cranks and can’t (easily) disengage except at the top of your stroke.
2s and 3s are the same size so no good for me therefore not tried them but have pins so will prevent the riding on glass platforms feel of the 1s.
Mallet dh are lovely, wider pedal so proper movement, platform surface is closer to the mechanism height so actually supports your foot. Take care as high pins + clipped in is (near) certain death.
Tried shimano dx types, platform was too ssmall for me and they don’t play well with my winter boots – lake mx140
No clipless pedals will work as flats, the mechanism is where your weight wants to be and is not grippy without cleats clipped into it.
The mallet dh gives a reasonable amount of grip when not clipped in for a few pedal strokes or what not when remounting but it’s not comfortable for any length of time due to the mech sticking into your foot, nor is it enough to ride anything which is intimidating enough to make you want to unclip.
For me they’re more comfortable than non platform pedals (which give me foot cramp usually) but they are in no way a replacement for flats.