I think there is no “one size fits all”.
I am very familiar with PhotoMechanic, Fast Raw Viewer and have just purchased Gnarbox 2. My personal photography style (landscape,street, travel with occasional sport) means I do not need to pre prune images before import ( except sometimes I want to import jpg only, sometimes raw only and sometimes for specific dates). lr is not helpful here.
As a pre processor for Lr, PhotoMechanic(PM) is the most mature and has a lot more functionality than most people need. It does allow you to preselect from an sd card and provides lots of tools to assist the workflow of a sports shooter or someone who needs to get the best images quickly to a client or editor, but PM learning curve and or pricing might be barriers.
Fast Raw Viewer (FRV) has very effective functionality for preselecting images. Further, they are extremely responsive to end user suggestions. They have implemented some of my suggestions already and have indicated they will add others in due course. When they do it will make FRV my favourite tool as a preprocessor to Lr (ie solve the issues I raise in the brackets above).
It is too early for me to comment on GnarBox2, but I can see the attractions of the device to support many of the workflow needs of photographers in the field, where backing up their images safely and quickly is vip. The ability to link to this device with smart phone or ipad provides options to rate and review and preselect what i ages get copies to your main workstation.
I have first hand experience of wild life photographers, shooting at 30 frames per second, who desperately need a solution to preselect the keepers before they reach the main workstation, otherwise the volume of data completely overwhelms the available storage.
Regularly wildlife photographers will have a whole string of images, maybe 30 maybe 100 that they know just do not make the grade and can be screened out of the workflow at the earliest point, preferably before landing on your main disk storage.
The ultimate solution is proportional to the volume of images you need to handle, the turnaround time and other factors.
While I am familiar with these options, I am not an expert, but will be happy to try and answer any questions anyone might have.