I think Risen 3 did improve in a few ways, although I'm not sure if it's a better overall experience than Risen 2. They added swimming which was really cool, and the exploration is better.
What hurt Risen 3 for me was that it just felt kind of generic. That, and it was too long imo. (3 is longer than 2 not vice versa.) When I got to the last island I remember being glad it was almost over, and I was expecting it would just be the end boss. Then it turned out to be another full island and a real slog too.
Yes, you are right, I checked my steam times and they are about 80 h for Risen 3 and 40 for Risen 2. So may be my subjective impression that Risen 3 was shorter came from the "genericness" you mention: The story of R2 was stronger and so there was more to remember and therefore I felt it was longer.
Some things in the game mechanics were indeed better in R3, not only the swimming, but also the fighting mechanics in my view. E.g. after dodging (rolling) it was much easier to get focus of the enemy again than in R2, where you needed to clumsily turn around before getting focus again. So there was some technical improvement in R3 compared to R2.
One thing was, however, odd for me: R2 gave a much higher resolution (3840*2160 versus 1980*1080 in R3) in the default settings and even if I choose the same resolution in both, R2 looked more detailed than R3. May be, that was again subjective or does anybody know of another explanation? I know that there was a high resolution patch for R2 once but I would assume that was only an upgrade to the quality of R3 and not something even better???
Still another observation: Normally I vastly prefer ranged fighting or magic fighting to melee and both were quite viable in the Gothics. In Risen 2 and 3, however, I found that in most fights you are forced into melee and have no real chance to get into ranged fighting distance, which was possible in Gothic. Don't remember, how this was in Risen 1.
Now, after so much talking I am tempted to try Risen 1 once again…