The flexibility and brittleness of your creation is really up to you as the creator. You can make pieces thicker by going back and forth and joining the strands, or you go very slow to make a thick strand that will be less flexible. This allows you to make stronger doodles that are able to act as supporting structures.
In the converse, you’ll create more brittle strands if you move the pen faster. The benefit of this is that thinner strands tend to be very flexible. If thinner, they are not easy to break, but they cannot support other objects.
So, in short, it’s up to you as a user to use technique and skill (which you’ll develop over time) to determine what you want from your 3Doodles.