That's a tough question and hard to prove. But few design choices makes it more (if not most) secure than other competitors:
Data is only stored on your device. No data is stored on our servers - no way to steal it (NOTE: Data shared with other websites could be compromised by weak security of the website, so please be conscious about who you share your data with)
Data is encrypted. All information stored on your device is encrypted using the latest algorithms and shared using secure connections only. So it is encrypted even when it is sent to other websites so no middle man can read it. Only you and the website you shared the data with can access it.
We believe that the fewer hackers know about the internals and encryption methodologies the securer our system. Although One account is not open-sourced we are using many external white hackers and other related services to find any vulnerabilities that may exist in our system before anyone else finds them and prevent any possible attacks.