Advantages:
- Let's you choose your own network provider(s) in a country
- Let's you take advantage of a network's specials (for example a cheap 20GB data bundle)
- Let's you switch network providers by visiting a website/app instead of changing sim cards.
- The networks still get their full rates
Disadvantages / Problems: I'm sure there are some so feel free to add. The main obstacle that I can think of is that you might have to contact and create relationships with all the network providers around the globe, which may be a very political process. I don't have much knowledge in this area but maybe someone in the mobile industry could fill in the possible setbacks and problems related to the mobile industry.