If I simply said 'it's impossible' in the first sentence of this article there wouldn't be much left to read. We love them, we use them, we want to work with them, but they can never be mastered 'perfectly', although it's worth it to strive for perfection at any cost. There's plenty of factors keeping us from unlocking full potential of being in compliance with methodologies.
First things first - the human. It's a rather large and important factor regardless of the methodology. Each member of the team has his or her own preferences, experiences, visions and expectations of the possible outcome of their work. It can strongly affect the final product. It's never easy to manage a team of invidivuals - even if they work as a 'team' - but flexible management of their tasks and ensuring that the workplace environment's conditions are met seems to make it much, much easier. Project Manager's job is to keep everyone involved, no matter the experience of the team members. Having everyone discuss the on-going development, future releases and changes, past decisions and present struggles is a win-win scenario for both, the team and the methodology of their chosing.
Secondly - the time. Working in a full compliance with methodologies requires time from both participating parties. Usually, the time factor is being cut short by the project owner and investor. Whether it's a MVP or a complete product the time is of the essence. It's important to hit the market before the competitors do. This forces the development teams to choose between maintaining the quality of the product itself or to follow the methodologies at all costs. More often than not, the decision is to bring the product to the client quicker. However, there are times where the project timeline and the development fully focuses on combining both, the human, the time and methodology. It's a dream come true for a Project Manager, the development team and - by the time the project is finished - the customer.
Working in pure compliance with the methodologies will never be easy as the working team, conditions and customers change. The deadline and budget also play a big role. Either way, regardless of these factors, staying close to methodologies objectives is rewarding and satisfactory, every team should be there or thereabouts at all times during their work.