The open access journal Scientific Reports publishes original research in all areas of the natural and clinical sciences. That's broad; it's a mega journal. With such a broad scope, I initially thought it a predatory journal, but it's published by Nature, so it isn't. What are the (dis)advantages of publishing original research in such a "mega journal"?
The main factor here is the broad scope. It can really go both ways and could be either a pro or a con. The pro is that potentially a very large audience could be exposed to your work. The flip side to this is that because of the broad scope, people in any one field - and specifically those the field of your paper - will usually not regularly go through the papers in this journal (in contrast to a specialized journal). So you may have a case where you are trading off exposure to people in your field for broader exposure to other fields - but of course this can vary for any specific paper.
Another pro of a broad journal is that it may fit interdisciplinary papers that do not otherwise fit in specialized journals. A potential con would be that there is less chance of getting an editor that is an expert in the topic of your paper.
In my opinion there are almost no disadvantages here compared to similar well received and indexed journals.
The usual argument would be that people who read this journal aren't from this field, so it might not be as interesting to them and people within that field might rather read the specialized journals and might miss it so you get less exposure.
I think this might have been true several years ago or even more some 30+ years ago. However, today there are so many publications every week, even within specialized subfields, that it's basically impossible to read all the journals relevant to your field or even just the biggest ones. Most people I know (including leading researchers) have given up on reading certain journals. What people do instead is creating alerts for certain subjects and frequently share intersting new articles which means as long as the journal falls into the index they use they will find it anyways.