Unless PI feels there's something about the output of a generator and/or transfer switch that'll make the EMS-HW50C malfunction, I don't see the point of installing it just to protect shore power. Generators and transfer switches can go bad just like anything else.
We installed ours between the transfer switch and the 120v control/breaker panel. Installed here, it protects our trailer from any shore power, generator, and/or transfer switch output issues. Granted, this does leave the transfer switch unprotected from shore power issues. However, since these are easy to replace and cost less than $150 (WFCO, Progressive Dynamics, etc.), for us, that's a fair trade-off vs. leaving our whole trailer vulnerable to generator (and transfer switch) voltage/wiring issues which can potentially damage thousands of dollars of items in the trailer.
FWIW, our EMS-HW50C has been operating flawlessly for a year and a half. We've used our generator for over 100 hours with its power routed through the EMS-HW50C with zero issues.