It depends on what you think is structure and its purpose. The biggest structure is the pond bottom.

To much plant structure can cause several types of problems from predation/balance problems to water quality issues.

To much wood structure can cause water quality (DO) problems as it decays. Same for to much brush.

On the fishing end the structure is to concentrate fish. If you have to much structure that purpose will be lost.

Over time a serious excessive structure amounts can cause plasticity or adaptation problems in fish species.