In truth, it's very difficult to compare learning techniques for effectiveness, and it's not always meaningful to ask "which technique is better?" As you note, it's possible for different techniques to have different benefits, and often they complement one another.
Studies comparing techniques and strategies need to measure the techniques somehow, and often "effectiveness" is measured in a single way. Even when effectiveness is measured in more than one way, studies are unable to cover all of the possible benefits. This is the reason that multiple studies and meta-studies are needed to make any useful comparison. Actually, in the field of language learning, I don't believe many questions have been really thoroughly investigated.
To give an example, if we are looking at the effectiveness of methods for learning vocabulary, there are so many ways they can be beneficial:
- Are you learning the word for the first time, or strengthening knowledge of a partially-known word, or re-memorizing a forgotten word?
- Does the technique build passive vocabulary (words you can understand) more or active vocabulary (words you can use)?
- Does the method build understanding of how the word is used in context? For example, what registers / domains it appears in, what collocations it is a part of, what specific meanings does it have.
- Does it build receptive knowledge or productive knowledge, or both?
- Does the method have a stronger impact on short-term or long-term memory?
So saying one method is superior to another is a difficult statement to justify. Word cards might be good for learning words for the first time, while extensive reading might be more effective for giving a deeper understanding of the word.
I think it's important to keep this in mind when reading answers on this site. Many questions ask for evidence from research, but in most cases the research will be too limited to make any definite conclusions. An answer which cites peer-reviewed research may look good, but if it's a single study that's not very generalizable, it might be less useful than a detailed analysis based on experience.