What are the advantages and downsides of a broad PhD thesis?
There are dissertations of either kind: Those which are clearly focused on one topic, and those which cover a wide range of different topic under some rather loose &quot;umbrella title&quot;.
Clearly, the traditional approach is the former. But the latter is rather common these days.
Whether the latter is acceptable depends on your committee, and thus to a large extent on the advisor who writes the report and sits on the committee.  If that person/those persons agree, there should be no problem. Overall, as far as I can tell the latter should be no problem these days, as long as the overall scientific output is on par.
On the other hand, you should take into account that looking in too many directions at the same time might be detrimental to your PhD in a different way, if its consequence is that you explore many directions but complete none.  In that sense, staying more focused is a &quot;safer&quot; option.  On the other hand, a PhD is a good time to exlore different directions, if you make sure that you don't use this as an excuse not to get projects finished.