Hi dodeqaa, it looks like some of your surface have a little bit of a slant to them instead of being directly in line with a world axis.

For example this one here:



If I zoom in a bit on a side view I can see the surface, if it was tightly aligned with the mirror direction it would only show the edge from the side and not some of the surface as well:



So the problem that creates with the booleans is that it's difficult to intersect surfaces that are barely grazing each other with overlapping surface areas.

= Michael