We're optimized for giving disadvantaged entrepreneurs in the world's poorest places the chance to improve their lives, not for storing financial assets. I think we make that distinction abundantly clear to prospective lenders. See for example our first FAQ for lenders, "Is Zidisha a safe place to store my savings?" (https://www.zidisha.org/microfinance/faq.html), or the first paragraph of our Trust and Security page (https://www.zidisha.org/index.php?p=123).
That said, repayment rates at Zidisha are increasing steadily as we continuously improve our lending model in response to experience. The 18% writeoff rate includes mostly loans that were issued over a year ago (which have had time to mature).
Our credit risk control mechanisms have improved dramatically since that time, such that I think it's reasonable to expect that the value of lending funds can be maintained over time without erosion for the loans that are being issued today. That said, Zidisha is still not a safe place to store funds one cannot afford to lose, and we do not market it as such.