It use a centralized server to handle transactions. – Bitpie Blockchain Wallet Review

Bitpie is not a real blockchain wallet. It uses the centralized server to handle all transactions. That means that when the bitpie server (getcai.com) is down, no one can send their coins anymore. You can use some network tools to check it (e.g. surge in iOS—block the domain *.getcai.com). You will see you cannot do anything for your coins. By the way, they also have the privacy issues, but they are not what I mainly discuss.
Review by Ervertin on Bitpie Blockchain Wallet.