Hi Billa, shapes like this which come fairly close to being smooth to each other but are actually just slightly sharp tend to be difficult to fillet:



That's because fillets shrink down to a point in a spot that is smooth. So for a spot that is close to being smooth it will shrink down to a small size there and be difficult to extend and intersect with surrounding faces.

It would probably be good to have this as one long smooth surface rather than a patchwork:



There is a fair amount of warping on the patched connection pieces.

A couple of other things are that it's harder for the filleter to process things when you're only filleting some of the edges that come out from a shared endpoint instead of all of the edges because then it has to try an intersect the fillets with each other. When all of the sharp edges coming out from one point are being filleted it will be able to make a corner juncture patch for that spot rather than having to deal with fillets directly running into each other.

One more thing too is that it's good for areas that are coplanar to be made up of just one big plane instead of multiple coplanar fragments.

- Michael