Install OpenCV from source or via Pip?
I will list out the differences between both
1.
Installation using pip
Installation is done at the default location where all the python packages resides.
Installation from Source
Installation location is provided by the developer.
2.
Installation using pip
In terms of performance, the packages installed might run slower because of the hidden conflicts between features.
Installation from Source
The developer can select the optimization flags during the compilation of packages which are responsible for the fast performance of library.
3.
Installation using pip
The developers can neither add nor remove features provided in the installation done by pip.
Installation from Source
The developer has all the rights to add or remove the features during the installation of library.
4.
Installation using pip
The package manager will do the work on behalf of developer. Package Manager is also responsible for taking care of library updation.
Installation from Source
The developers are responsible for  feature selection and updation of library. They must be aware of new package updates, latest security patches etc, to keep themselves updated about  the library.
Hope this helps you!
I've seen 2 ways of installing OpenCV (there might be more ways which I don't know): 

Installing from the source
Installing with pip: pip install opencv-python

My question is, why we need to install OpenCV from the source while we can simply install it using pip? Since people are using both of them, both must be useful. If so, there are any conditions for selecting one of them?