Most every design has a comparative, whether stated or not. Making broad connections internally and externally are natural and add contextualization.

David Coker But why it is unethical? I mean to compare each data set is a good attempt because it adds depth and breadth to a research project. For me, I compare my data in my mixed-method study, which turned out to be amazing. Maybe I guess it is because of different paradigms that we adopt? IDK...

Who says it is unethical? I never hear that complaint. Logical is a much more likely question. When we compare, we must consider the different contexts and how the variables where defined and measured.

David Coker This is the original sentence, comparative designs may bear too many echoes of experimental research and are seen as antithetical to qualitative inquiry (Clark et al, 2019).

Make sure you read the entire paper. Some disagree, wording to Clarke et al. (2019). There is the danger in any comparison that one is unfair and holds one phenomenon up to another. Context, variables, and concepts are specific to situations. Ethnocentrism is one such problem, especially when one makes overt value judgments. Yet, Clarke et al. (2019) answer the question you pose: "Further, we would argue the value of a comparative design is that it allows for a more nuanced understanding of how a particular phenomenon is socially constructed."

David Coker Indeed, I think it is a fascinating argument.