My observation is that Scrum/Agile/<your favorite SDLC methodology here> is just a proxy for team competency. If you have a team that is technically proficient at what they do, have reasonable time management skills, and are motivated and incentivized properly, it doesn't matter what SDLC they use and they'll figure out an efficient way to succeed. At the same time, if you have a rag-tag team of developers who don't know what they're doing all the time, it doesn't matter what SDLC they are using, they are going to fail. SDLC might give some guidelines for managers to feel safe but it all comes down to understanding the economics of the team and figuring out what works best for them.