What are downsides to using septal cartilage to build a tip? What are complications?

Ideally, rhinoplasty requires structural support to allow a long-lasting outcome. My preference is always to use one's own cartilage and avoid synthetic material such as medpor. Typically the septal cartilage is an ideal option, if available, for tip rhinoplasty. The ear and rib cartilage can also be used depending on the location that the cartilage is being used at. Septum and rib cartilage give the best structural support. Ear cartilage can be used on the outer tip cartilage or "crushed" and "diced" for other regions. Septal cartilage is unlikely to "curl" up or cause collapse if used correctly.

Revision rhinoplasty and tip-plasty often uses grafts. The most commonly used, and often preferred, are your own cartilage. The source depends on what's left from prior surgery as well as what is being rebuilt. I tend to prefer septal cartilage first, ear cartilage second and rib cartilage third. However, the amount of cartilage necessary and what we find left in your nose will be the final determination of what we use. This can be discussed and determined at the time of a personal exam where we can look at and feel all parts of your nose.

I think that an evaluation in person is necessary to get  better appreciation for what you need. There are several donor sites for cartilage: ear, septum, and rib.

Septum is a great source of cartilage.  If you have medpor in your nose, I think getting it out is not a bad idea.  Septum is the gold standard for cartilage grafting and I recommend this approach.  Other sources of cartilage include rib and and ear, but if septum is present, go there first.  The medpor can sometimes be a problem long term.  Good luck