One could also say that you now have two potential points of failure for GPS... And the additional cost of buying the second one in the first place. Similar to single vs. twin...
The Lynx is an excellent unit, but doesn't make as much sense if your navigator is Garmin, because you can't display the weather from the Lynx on the Garmin, only the traffic. If you have someone else's navigator, sure, but if you've bought into the Garmin ecosystem, there are some advantages to staying with Garmin.
Also, the Lynx really needs to be installed in a visible position for it to have any advantage at all, and that means moving everything else around, which is expensive if you weren't doing that already.