<Some question about design or product>
What are the pros and cons?
Sometimes, these questions get closed as 'too broad' or 'primarily opinion'. Why is that?
The "what are the pros and cons" style questions are slightly obscured polling questions... though only very slightly.
Questions of this type sometimes have a dozen answers that look like:
- Point 1
- Point 2
- Counter point 1
- Counter point 2
Something no one mentioned yet is that Foo is really good at doing bar.
The answers to these questions are a poll asking for people to collect various bits of facts into what they think (opinion - a pro for one person is a con for another) into a post. As there are often many of these factoids that goes into making a decision, there can be scores of them (just think of the weighted decision matrix that managers so love - I once had to fill out my opinions on some competing products that was over 100 rows - each with a different factoid of some importance (and I feel it still left out some important items)).
The nature of these questions also makes it so that they are never complete. One can never have an authoritative answer for the pros and cons, nor can one have an answer that includes everything - someone will always find another point that they like or dislike about the design decision. Questions of this format from three years ago still get people adding single pro or con items to it (often just a single sentence).
These types of questions rarely produce good answers and instead seem to have shorter and shorter answers as time goes on.
When comparing products, this falls into a further problematic area of shopping questions.
What are the pros and cons of having a Linux dev system rather than a FreeBSD dev system?
Design decisions are slightly better, though they are still problematic. They are still in the range of 'too broad' and 'primarily opinion'. A better option than asking for pros and cons is to look at trying to write the question as a design review.
Realize though, that asking for a design review rather than just 'pros and cons' isn't a simple matter of changing the finishing up part of the question to ask for a design review. The guidance on how to ask a design review question can be found at Are Design Review questions on-topic?
Many of the questions that have been asked in the 'pros and cons' format in the past can fairly easily be rewritten in a way that isn't a poll and instead identifies the problem and asks for solutions to it rather than polling for tidbits of opinions.
That said, many of the historical questions have answers that are in the 'pros and cons' format that would make rewriting the question while maintaining the validity of the answers difficult.
Not sure you noticed, but your question is one-half of a pros-and-cons question: You are asking "What are the cons of pros-and-cons questions?"
Anyway, I would argue that such questions are not a problem as such. As @Trilarion notes: A focused pros-and-cons question can generate useful discussion that's very educational to site visitors who find it.
With an unfocused, overly wide-scope or overly vague pros-and-cons question, the issues user40980 mentioned do become problematic and can make the question-and-answers not useful.
Caveat: I'm biased, occasionally having asked pros-and-cons questions myself.