Should a criminal lawyer, for example, have a solid background in biological sciences and ballistics? Or is it enough that he or she understands criminal law?
No one can be an expert in everything. Try and you'll likely end up mediocre at many things.
Like a general practicioner in medicine, lawyers need to know their field, have a working knowledge of the germane aspects of other fields, and to know when to call in an expert. It is my understanding that criminal lawyers routinely call in expert (subject) witnesses, both to inform the court and to provide a more objective opinion than the counsel of either side could.
Understanding criminal law is no mean feat, especially if we use understanding in the sense of "have it in my mind and can readily apply the relevant statutes to this case".
Tellingly, there is no prerequisite coursework for law school. If a layer had to master a body of knowledge outside the law to practice, then that other body of knowledge would be required for admission to law school or taught during law school. The exception which demonstrates this heuristic is patent law.
This question can, in some ways, be extended to anyone who is at the intersection of several disciplines - be that lawyers, some types of researchers, etc.
So I'm going to give my answer in more general terms.
"Do you have to be an expert in everything that touches your field?" - No.
That's why we have experts. Doctors call in experts for consultations. Lawyers have expert witnesses. In my own work, I commonly find myself collaborating with experts in particular fields while working toward a more general problem. What is important is that you, the non-expert integrating their expertise, needs to be familiar enough with the ideas that you can critically evaluate what the expert said, ask questions, and not view their field as a black box for which answers come out that you just have to trust.
What, and how much, expertise is required for a particular field of law will vary very much by the type of law. For example, patent law likely requires a fairly extensive array of expertise (and indeed, I've seen it suggested that a degree in a technical field is a bonus for getting into patent law), financial law requires at least a passing understanding of accounting, but a small town sole practitioner is likely better served by knowing the name of the county clerk's children, and how the local sports team is doing.