Hi Lemo, thanks for posting the file, that makes it much easier to help you.

MoI is not able to do a good job of automatically matching curves for lofting when there are different number of segments. It can easily happen that a piece of one curve is mapped to attach to the piece of the next curve around a corner, causing a kind of torquing or twisting effect. That's what causes the kind of wild surfaces that you are seeing.

To get a good result in this situation, you will need to prepare the curves a little more before doing the loft.

To do this, I selected all your curves and then ran Edit / Separate to break them into their individual segments. Then I clicked on them to examine the structure of the segments.

The goal is to split longer segments up so that there are an equal number of segments between all curves.

To split a curve, select it and then run Edit / Trim. At the "Select cutting objects" prompt, push the "Add trim points" button - this allows you to pick points on the curve to act as cutters, instead of picking other objects in the scene like other lines or curves. After you pick the "Add trim points" button you can now click on the curve to place points for where you want to split it, and then right-click or push "Done". Then at the next prompt you can pick pieces you wish to discard - in this case you want to keep everything, so right-click or push "Done" there again instead of picking anything.

Using that process you can split the curves up at strategic areas to make a better mapping between the different segments.

So for the outside smooth curve, I added trim points at these locations to split it up:



Each of those points correspond to the endpoint of a segment on one of the offset curves. Also the middle curve needs to be split up at one point in the lower-right:



After performing that splitting, there are now an equal number of segments between all curves - this means that MoI will connect each segment directly with one another. So now you can select all the segments, and use Join to make them into longer curves, and then use Loft on them to get this much improved result:



The adjusted curves are attached here as loft_segmented_fixed.zip

Basically right now MoI does not have the kind of intelligence at picking those kind of alignment split points as a human does, you need to help it out by giving it more structured input.

Sometimes you may have a different number of segments because some pieces of the offset curve are completely eaten away. When this happens then you have to work by doing Lofts in smaller pieces at a time instead of joining all the curves together and doing one big Loft. But if there are an equal number of segments one big Loft is ok.

Hope this helps!

- Michael