Some researchers argue that case studies cannot be replicated and cannot be used for prediction so the results cannot be generalized. Also, small sample sizes in case studies cannot be generalized as it is not statistically representative of the population.
However, there is also arguments for its generalisability:
There can be some generalisabililty for different case studies that are similar, in order to compare results of each.
Case studies can be generalised through setting, however, studies should never take data or results from a single case study unless the experiment is very similar/the same.
Single case studies have theoretical generalization but cannot be representative to the entire population. Theoretical generalization refers to whether the results of a single case study can be generalized to existing theory.
A single case study can be used for inferential generalization, which means that the findings of the case can applicable to similar settings and situations, but only when the researcher has provided a description of the phenomenon and the context. This is also called transferability.