I was working with a student recently and was reminded of all the subtleties of uke strumming. For instance, I use my index finger, sometimes supported by my thumb, to strum. When I do a downstroke, my index finger is curled upward and the nail part of my finger drags against the strings. When I do a return upstroke, my finger points downward* and the flesh of the finger strums the strings. Thus the first stroke has a harder, nail driven sound and the upstroke a softer, flesh-driven sound.
*This isn’t quite right. On the upstroke, the finger starts out pointing downward but really kind of travels upward during the course of the strum. This curling action is part of the strum.
As I observe thumb players I find the reverse is true. They strum downward with the flesh of the up-pointing thumb and upward with the nail of the down-pointing thumb.
Just this fluctuation between nail and flesh gives a strum a specific flavor.
This video demonstrates the different sounds from nail and flesh (though that’s not the purpose of the video.)
Of course another factor is where you strum. I play a lot of baritone uke and tend to go for the soundhole. However, advice is different for different ukes. I nod to this site which says…
The ‘sweet spot’ on the soprano and concert ukes are around the point where the neck hits the body. The sweet spot for tenors is a little closer to the bridge than this.
There’s more useful info at the link including discussion on different strumming techniques like chunking, and demonstrations of many strum patterns.