Lack of transparency in terms of use can lead to vendor lock-in. Contracts and SLAs should clearly define the relationship between Cloud Service Provider(CSP)and the cloud customer. Clause of data portability should be there.

In a shared responsibility model. Data security is responsibility of the cloud consumer and he is legally liable.

APIs are typically REST for cloud services, since REST is easy to implement across the Internet. REST APIs have become the standard for web-based services since they run over Hl'-P/S and thus work well across diverse environments.
Reference: CSA Security GuidelinesV.4 (reproduced here for the educational purpose)