Fair appraisal, based on experience.
I come from a background of other outdoor activities and have used Garmin devices for navigation & messaging, culminating in my current Garmin 66i (combination navigation GPS + inReach messenger). The 66i does everything well enough for me, except the screen is too small for me to easily view it on a bike. I bought it more as a replacement for an older navigation unit and an inReach, not for motorcycle navigation.
Even with a good isolation mount, I don't want to hang my primary phone on the bars. With that in mind, having something dedicated to moto-nav would be nice, which has led me to considering the questions of this thread. Ideally, my device would allow the sue of a dedicated keyboard when off the bike, and would perform other functions while I travel. I value what a modern Garmin device offers, but it's not going to perform as anything other than a navigation aid (and possibly a satellite messenger). Not that there's anything wrong with that.
One thing to note about using older phones or tablets is that even if the screen is useful outside, the operating system may be a limiting factor. For instance,
Android Kitkat (v4.4) is the oldest OS supported by the Google Play store.
Android Marshmallow (v6.0) was a major upgrade that can impact apps that rely heavily on storage (like offline maps) because it allows better use of secondary storage media. There are other lines of demarcation in the
history of Android that can impact a device's usefulness as a moto-nav unit, but those are two standouts.
iOS follows a similar pattern.