Problem seems to be that in Sierra the nvda_drv=1 boot flag does not work, you need to have the nvda_drv=1 in your nvram. The nVidia Web Driver adds this nvram variable, but if your nvram is not working properly (changes are not permanent), this will setting be lost upon reboot.
Some motherboards (notably all Asus Z97 boards!) has nvram issues with current Clover versions - see
https://sourceforge.net/p/cloverefiboot/tickets/165/
This was not a serious issue in the past, but now it prohibits the web drivers to load. You need to set up Clover to use a file based nvram until this is fixed (create nvram.plist in your root, add nvda_drv=1 to it and set up the EmuVariableUefi Clover driver).