Coming across this message and the only way forward being to click 'More info' is off putting and may be stopping people from downloading the Flex Revit Add-In. See user experience screenshots attached.
From release 1.2.51 of the Revit addin onwards the installer is signed with an EV certificate (rather than the OV certificate) so the Smartscreen filter should not be triggered.
We also no longer need to wrap the MSI in a zip to stop browsers warning, so the MSI is now downloaded by default.
I've retrospectively re-signed build 1.2.50, which is the current release.