There is a lot of them but most of them suck. But among the god awful ones there is some great ones which put into the right situation can be very entertaining.
The point of these characters is to point out how dangerous something is whilst making the hero of the story look brave for attempting something reckless.
Another thing about these character is they will always question the logic of everything especially when magic or complex science is involved. Watanuki from xxxHolic is a great example of this. In the beginning when he is introduced to spirits and magic he simply chooses to ignore them hoping hey will go away. Being an intelligent guy he knows logically ghost shouldn’t exist but the hard fact is ghosts are around him making it hard for him to deny them. He matures later in the series but he is constantly pointing out why magic is crazy and unrealistic.
Usopp and Chopper from One Piece also fall into this character trope. Usopp’s main personallity trait is that he is cowardly but this relfects mostly when he is presented with otherwordly situations that should be impossible. That and over powered villians. Seriously who wouldn’t be scared of someone made of lightning or darkness. Chopper simply plays off Usopp’s cowardice coming across as even more cowardly at times.
Lets not forget the ultimate coward Jacuzzi Splot from Baccano. He is cowardly in a very unique way but over the duration of the anime he shows he has enough courage to face a greater opponent.
The draw back to a cowardly character is that they are often poorly characterized and simply remain cowardly right until the end never evolving past the whining and bickering which after a while can drive you insane. However whining and bickering can suprise you sometimes. Every charater in Infinite Ryvius does this but it was still an amazing story.
I guess it all comes down to whether or not the writer intends the coward or realist to be a minor character or a major character that evolves or the length of the story.
Could anyone come up with other examples? I was struggling a bit this time.