This is great feedback, and you're totally right about your understanding of the problem. We do rely on having a full picture of all your exchanges and wallets for all the calculations to work correctly. For crypto wallets that we don't support, we recommend for now manually entering the transactions from those wallets. We plan to support more wallets in the near future. We have some more info on this issue in our FAQ: https://www.cointracker.io/faq#cost-basis-wrong

We will also make it clearer to the user when we see there are outgoing transactions that may indicate a missing wallet / exchange.