If you run a marketing/design agency, all you need is Kanban and a few techniques from traditional project management. Scrum can also work (like when you're working on a long-term project), but that's very rare because most agency contracts are finished after three iterations, and the client won't like the idea of getting half-finished work and having to wait while you iterate.
If you run a development agency or a consultancy, you can use Kanban, Scrum, or XP, with this caveat: if you work on short-term, on-and-off projects, Scrum and XP will be a tough sell for the client (although you can still use some best practices from each methodology).
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If you learn and understand the methodology/framework, adapt it to fit your needs, and roll it out properly, you'll end up with something valuable. The main problem with methodologies happens when companies jump on board because something is a hot topic at the moment. Many forget that implementing a methodology/framework takes years of hard work.
For example, the reason a lot of people think Scrum is not good is that a lot of companies jump into it head first, buy a ton of "Scrum products," and force everybody to change their terminology. Then things get broken, while the company keeps bragging about being a Scrum shop.
Implementing a methodology/framework in your project management deliverables does not mean throwing all your existing processes out the window. Instead, it's about slowly changing company culture and how you think and work.
Note: when you hear someone uses waterfall or agile methodology, it doesn't mean they use a methodology called waterfall - Instead, they use the word “waterfall” as a shorthand or an adjective, meaning they plan the whole project upfront and then execute one article at a time.
In a nutshell, you won't go wrong with Kanban + a few traditional project management techniques, no matter what type of project you have. Further, if you work on big, complex, and long-term software projects, you can expand your knowledge and adapt to Scrum or XP to boost productivity even more.