I did a quick test of an old RF filter file I had using AC Power sources and S-parameter analysis. The two Sources P1 and P2 were rotated 90 deg. Another problem that is not a real issue for QucsStudio is a formula for Group Delay used in older Qucs had used the math function “unwrap” that QucsStudio didn’t know. I’m sure there are other issues as well.
I have a huge number of Project files for Qucs, Qucs-S and QucsStudio. Some are the same schematics solved using the three different versions of “Qucs”