Hi paoki, welcome to the forum!

If you've got surfaces that just touch each other edge-to-edge then try using the Edit/Join command to glue those together into a solid, rather than the booleans.

The Booleans are more targeted towards cutting the surfaces with one another as part of the process, which can make things more complicated in some situations.

Join on the other hand is only focused on gluing edges together and not on cutting objects with one another.

Do you have an example .3dm file of something that is causing you problems? If so, can you please post it as an attachment here? That will help me to take a look at your specific situation.

- Michael