Hi MRAKGR, I can't really answer your question very well without being able to examine the actual geometry instead of only a screenshot.

It helps a lot to post the .3dm model file with your question instead of only screenshots.

In general it's true that it can be more difficult for booleans to process objects that have overlapping surface areas. The reason why is just that it's a fundamentally more difficult process to get clean intersection curves with such objects.

The way booleans function is that intersection curves are generated between objects and then those curves divide the objects into separate chunks. To do that the curves must make well formed closed loops. If the intersection result doesn't do that and is messy then it can prevent the boolean from working.

Overlapping areas can be ok in some cases if they are very precise in the area of overlap.

The easiest way to help ensure that you'll get clean intersection curves is to extend objects like you did, so that they are pushing through each other by some margin instead of having surfaces that skim over each other with overlapping surface areas.

- Michael