What concerns lie ahead as more organizations move to public cloud?

The biggest concerns regarding public cloud in my opinion are the following:

Controlling costs
Maintaining security
Lack of central governance

Security, Cost and Compliance.

One of they key issues will be the need to retrain the existing Networks, Infrastructure, Cyber Security and Application dev team in a totally new conceptual way of thinking. Combines with the obvious skills shortage, these 2 factors will delay many organisations.
On the  Cybersecurity topic  - Most of the well-known security providers will have secure user identity management, authentication, and access control and these will need to be  integrated with monitoring tools and/or processes where possible.

Governance will be another key factor, who will audit Cloud systems t ensure that are correctly configures and security designed and implemented.
Compliance - When applications  migrate to a cloud environment  system or data moves from an on premise environment to the cloud, ensuring the organisations stays compliant with official regulations and laws such as PCI, GDPR must be considers as well as on going auditing and compliance checks, and who is responsible for data deletion and destruction.

The largest concern I have with all of the cloud providers is security and the opportunity for an outage caused by human error. With more and more of internet based business happening through these handful of providers, there is potential for major interruption in daily operations if something goes wrong. I know the cloud companies all tell us there is little room for error, but I think they are overestimating their capabilities and resiliency.

Security, especially during the transition process