Public Cloud has some advantages – generally a low upfront cost with practically infinite scalability.  But it has its disadvantages:

Service Levels ​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​– providers often have no SLAs at all so uptime, performance, service quality etc., fluctuate with user demand.

Network Dependence – performance is limited by the Internet, which may slow down at peak times, reducing or halting employee productivity.

Privacy Compliance​​​​​​​​​​​​​​​​​​​​​​​​ – it is next to impossible to be compliant with HIPAA, privacy, PCI Data Security Standard, Federal Rules of Civil Procedure, and audit requirements.

Vulnerability – there are no guarantees for timely patching, malware protection, security of confidential data, in-house administrator access restrictions, or audit trail recording.

Disaster Recovery​​​​​​​​​​​​​​ ​– with no control over physical hardware, clients do not have hardware redundancy or isolated backups, which can mean no DR capability at all.

Supplier Lock-in ​​​​​​​​​– public clouds tend to be proprietary, so switching providers is resource intensive and costly, and may even be out of the question.

Dependability​​​​​​​​​​​​​​ – if the cloud provider goes out of business you could lose access to your applications or data, and possibly even your backups.