I think the lack of certified mounting is what makes VFR GPS a backup
for flight in IMC, rather than a primary. Note that an IFR GPS includes
installation in it's design. They are installed to be part of the scan.
There may be VFR installations that are indeed in the scan and
convenient enough to use, but the typical VFR installation is lacking
in this department.

I have a King KLN90B in the panel and a Garmin 195 handheld. I do use
the Garmin when in the clouds, as a backup and for overall situational
awareness. In my case, my VFR Garmin is not nearly as good as my panel
mount King. Installation is the primary difference. There is also the
issue of training and just what you are supposed to do when. The King
manual has written procedures for flight in IMC/IFR environment, the
VFR unit does not.