The ROI is around 1-5% on BTC, ETH, EOS and so on.
More in the article: https://medium.com/okex-blog/ok-piggybank-gain-extra-with-your-spare-coins-1efe3c9b37cd
Do you know about some other platforms that offers something similiar.
Would you trust OKex to store your cryptocurrencies forever?
right . exchanges are online and expose to many risk . they can be hacked or worst they can turn into scam one day but im not totally saying they are , i just only found simillar cases that happend on the past . the idea of storing fund to earn interest is cool and yes you can treat it as a piggy bank because a piggy bank only means that you only put small or excess funds ( not your whole liftime saving ) so for me , its not a big deal overall .