When using the extension without specifying a value for ogp_image, local builds return the following error:

findfont: Font family 'Roboto' not found.
findfont: Font family 'Roboto' not found.
findfont: Font family 'Roboto' not found.
findfont: Font family 'Roboto' not found.
...

To reproduce, just create a default project with sphinx-quickstart and add the extension to the list in conf.py.
The error disappears when specifying a value for ogp_image.

Roboto is not installed on my system. I haven't tested if the error disappears when installing the font.