One of the ways that a question can be opinion-based is if someone has to make a lot of guesses, or a few very significant guesses in order to answer the question.
Since Guesses are in essence judgement calls made when too few facts are available, they are inherently opinion-based. Guesses suffer from the exact same problem that makes opinions bad. Answers based on guesses are not authoritative, ans since they are not authoritative, they are not very valuable.
One of the things I do when I evaluate a question, is I try to imagine how much guessing it must take in order to answer the question at hand.
If the question seems like it would take a lot of guessing, then I consider it to be too opinion-based, because people will have to guess too much in order to answer the question.
The question in the OP requires a lot of guesses to sort out thing like the definition of "advantage", and a whole bunch of other factors that make advantage or disadvantage actually gained from the system being 2-parties or not very unclear.
TLDR: the question is too complicated to completely answer with just facts, so opinions would be needed to provide an answer.