Stagg are stamped from the same pattern as Ohana, Mainland et al. However, there is a definite difference in the levels of attention to detail in things like cosmetic appointments such as binding, and in choices of hardware etc. Most of all the level of quality control varies a whole lot.
We have had two Stagg ukes - both at the top of the Stagg range of Mahogany ukes. I had a concert and my wife has the soprano. My wife's is very good. The tuners are cheap, but they're usable and otherwise it is a nice, loud, sweet sounding soprano. My concert on the other hand had serious issues with a misaligned bridge that made accurate intonation impossible until I made a pretty bizarre field modification to the bridge saddle.
I've played a couple of other Stagg sopranos and found them both slightly iffy. One had a bridge misaligned so that the strings were too close to the edge of the fretboard on one side, and the other just didn't sound too great.
On balance, if you get a good Stagg then they are not far away from Ohana and Mainland in basic quality of construction and sound, and can be great value. My wife's soprano is a very nice uke.
The downside is that, in my experience so far, the good Staggs are not as commonly found as the flawed ones.
