I think that diffusers that clip on the end of the flash are over-used and abused.
The ones that look like translucent plastic boxes on the end of the flash are designed to make the flash behave like a bare bulb, and bounce light everywhere.
So to see them used outdoors where all they do is cut down the flash seems odd.
It will lower the flash output by stopping it being directional, so it will appear less harsh and more fill-in than main source.
But it's of little smaller area than the front of the flash itself.

Of more interest to me are softboxes to go on the front of the flash. I use Lastolite boxes designed for flashgun. They pack flat and unfold and velcro on to the flash. These lightweight, portable, flash-through diffusers work well.
Again there is a loss of flash power.
For best even illumination you need to zoom the flash out to its widest, and that reduces the output while giving a nice soft light.

Gaffer tape and white cardboard works well as a reflector. Shape it so that it matches the spread of the flash beam, and tape it to the back of the flash head which you fire vertically.
It makes a large triangular reflector, but not a very efficient one. Funny shape catchlights too, if that matters. But if you carry some tape in your pocket and can pick up a sheet of something white, even paper, you can make a quick standby reflector.