The main advantages of using a Service Design approach I see are:
Service Design makes people hate you less: the idea of service design is to make services better for the humans who use and build them. So making life easier for many people sounds like a pretty cool thing, no?
Service Design gives you a toolbox: knowing the world of Service Design, gives you access to a lot of tools, methods, references, authors and knowledge that help you achieve the goal of better-serving others.
Service Design gives you a common language: if you use Service Design within a team or organisation it gives you a shared language and toolbox to speak about how to improve your service.
The main disadvantages of using a Service Design approach I see are:
Service Design kills your ego: as Service Design often is based on the feedback and ideas of others, it can be pretty hard to see that your ideas suck and that others have better ones.
Service Design makes you see all the shitty services: Once you better know how to create lovely experiences for other humans, you wonder why so many organizations still create very shitty experiences for you to go through.