The panel study has the issue of

Panel studies suffer from attrition, the progressive loss of respondents over repeated waves.

Drop-out can make the remaining sample unrepresentative.

A panel is not a single survey, since it revisits the same people over time.

It does not centre on different variables, as it tracks the same measures.

It studies people, not inanimate objects.

Panel conditioning, where repeated measurement changes responses, is a further problem.