Q: Why use one rather than the other?
Its important to first understand why different frameworks exist to scale scrum or agile teams. In my view, the answer is money. After the agile manifesto's publication in early 2000s and its subsequent fame, Scrum soon became the "market leader" in the world of agile. Scrum did not, theoritically at least, address how to deal with scaled setups. As a result, many organizations have emerged attempting to address this as well grasp the largest portion of the massive cake of agile development frameworks.
Scrum became the marker leader among agile frameworks for a number of reasons but, IMHO, simplicity and low barrier to learn and adopt were the most important.
This is exactly why I would encourage using Nexus. More on why Nexus below.
"Q: Does one cover more areas than the other?
The answer is yes if by more areas you mean more material. However, it is important to stick to basics of Scrum (empiricism) and the underlying principles of agile development when looking at all these frameworks.
It is critical you do Scrum/Agile right before you scale. If you must scale, I would recommend heading to Scrum.org and looking at Nexus but only if you are comfortable with Scrum. Scaling scrum doesn't solve your problems, it scales those problems.
So simply put, IMHO, there is no such thing as more areas.
Q: What is the "most popular" framework as of today?
Definitely SAFe. Well, at least in Europe and based on a quick search on Indeed for job vacancies for SAFe as compared to the other frameworks.
Q: What are their strengths or weaknesses?
There is a tendency to "scale" scrum every time things appear large. More often than not, scaling scrum is just unnecessary. For example, as you mentioned, when working on unrelated projects.
I have almost never seen a scaled scrum setup with the appropriate number of scrum masters involved (I have been involved with Scrum for only 5 or so years). This tells me that the lone Scrum Master with a growing team decided to use scaled scrum to continue being the "Scrum Manager", and not Scrum master. You cannot scale scrum when you are not even scrumming. I digress, however.
Without attempting to compile an exhaustive, and unnessary ist, I would say the biggest strengths of Nexus are:
- It beautifully extends Scrum by providing what Scrum's and Nexus' leading author Ken Schwaber calls an exoskeleton.
- It keeps the "material" at a minimum so its concise and fairly quick to learn.
- It has a low barrier to learn and get certified in.
Its weakness would be: Lack of widespread adoption given its relatively new (although the best, IMHO) and that means if you join a scaled Scrum setup, you may need to quickly learn the terms as used in other most widespread frameworks.
You can swap this as strenghts and weaknesses of the other "heavy on material" scaled scrum frameworks.
Q: Moreover, certification-wise, is any of these more "recognised" than the other?
As mentioned above, SAFe appears to be most in demand. However, I would strongly discourage you from joining an organization insisting on SAFe or CSM certification as opposed to one of the other Scrum certifications like PMI-ACP, PSM and so on.
This is actually a good indication of an organization not truly adopting the underlying values of Scrum, Agile or Lean. Where is the Kaizen or opennes in insisting on hiring SAFe certified as opposed to Nexus certified or simply Scrum certified for that matter.