Can it be considered a disadvantage that the candidate is only mastering new technologies and does not have deep knowledge in already familiar areas?

This is not necessarily a flaw, but rather a characteristic of the candidate's level of development. Learning new technologies is an important skill, especially in the rapidly changing IT environment. However, a lack of deep knowledge in familiar areas may indicate that the candidate has not yet reached maturity in those fields. For junior and middle levels, this is normal, as they are in the process of learning and gaining experience. The main thing is that the candidate demonstrates the ability to learn quickly and apply new knowledge, as well as a desire to deepen their understanding of already known technologies.