Fill between slices work well on many computers on all kinds of data sets, so there should be something unusual about your data or how you use the tool.
The best would be if you could share an example data set that you have trouble segmenting using this effect. If you cannot reproduce the issue on any public data set then you can post a screen capture video or a few screenshots and then we may see what’s wrong.
I agree with you about functionality, because I have worked with Slicer many times. The most interesting thing is that I worked with the same files literally yesterday and performed the same actions and everything worked perfectly, but today it doesn’t. Although all the “ingredients” are the same.