K2 is a great content construction kit (CCK) that makes it super easy to construct complex websites with complex content structures.
You can apply different templates to different categories and have some really interesting content displays, but for those that are using a very simple structure and don't need the extra bells and whistles that K2 provides, Joomla core articles is usually a really good fit and in most situations, users can do without the need of K2 altogether.
For some users, K2 is just too hard for them to grasp. The confusion between the Content menu and the Components->K2 menu is a usability issue if they haven't been overridden or replaced.
The idea of it was to give it extensive publishing features with a huge amount of flexibility but on the downside, the huge amount of flexibility can present users with so many options and parameters that they get overwhelmed and confused. In these sort of situations, Joomla articles is a better replacement, especially for small websites that more or less stay static for the existence.
With the the later features in Joomla 3, such as custom fields, and even more advanced features in Joomla 4 when it comes to templating, there will be a less of a need for components like K2 where core Joomla can handle all of these aspects itself.
K2 itself hasn't seen many new upgrades or features in since mid 2014. As a result we also feel as if it is falling behind in development and could hold back various websites from being upgraded and kept up to date. It becomes some what a risk for us to have sites continuously using it.
A few of our clients fall into this category and we have created a migration script that pulls in K2 items and converts them into Joomla categories and articles.
The K2 Migration Script
The migration script is under our BitBucket account where you can access it and customise it for you projects.
This is a server side PHP script that needs to be executed at command line in order for it to work.
It is not a Joomla extension with a nice visual migration interface.
If there is interest in that perhaps we will build it but at the moment, you will need to be a competent developer to understand how to use it and migrate your content.
Feedback and Support
Please feel free to comment and improve the migrator, we'll be more than happy to provide assistance in regards to using it and helping you with your Joomla / K2 site migrations.
We've used this very script to move many of our clients from K2 to core Joomla over the years and have improved upon it as needed.
Contact us if you have a large project and need a hand in regards to migrating your data from K2 to Joomla or another CMS altogether.