Is searching in article references considered a systematic way of literature search? Doing a systematic review, either quantitative or qualitative, requires developing a well-defined protocol as a method of conducting it. This includes defining the search methods used in the identification of eligible articles, which include two methods:

database search using search queries (a main method)
searching in the references and cited-by sections of the selected articles (which are already retrieved by the first method)

Let's assume that an author (of a large review) does their best to formulate the search queries to better represent the review question, but later they realized that considerable amount (e.g. 50% or more) of the relevant articles are found only by using the second method (searching in articles);
Is the second method less 'systematic' than the first one? Does it compromise how much 'systematic' the resulting review is?
If so, does the above scenario affect the validity of the search queries used (i.e. should they be reformulated to increase the recall of search and retrieve more of relevant articles)?
Edit: More extremely: If 90 out of selected 100 articles are found only by using the second method, how much does this affect the quality of the systematic review?
I would add to the other answer(s) that you could validate the first approach using the second approach. If your search criteria systematically miss relevant articles in the journals covered by the database, then that's a clear sign you need to reformulate your search criteria because they're clearly not sufficient.

The second thing to add is that any search should also try to find unpublished articles. These are definitely not going to be in databases, but may be cited in papers (thus the importance, at least in my view, of the second approach). It might be worth adding a third approach that involves search conference abstracts, where unpublished (especially recent) work is likely to be found.