I have not used the mobile app much, so I can't swear as to what data level they have there. For at home planning, I have used the web-app on a laptop. For that, Navionics claims "what you see is what you would get on the chip". Their FAQ directs customers debating the purchase/value to look at their online maps to be properly informed on what to expect. Go to:http://www.navionics.com/en/hotmaps-platinum-1#coverage
for a list of lakes covered. That list also indicates which ones have 1-foot contours (HD). Everything I see there matches the 2D view on my HotMaps Platinum chips. The 3D and satellite overlay are marginal features of the chip not available online. You are correct that you will get little data for small secondary lakes not on that list. HOWEVER, if you fish there a lot, you can build up your own private database of sonar scans over time with "SonarChart" - a feature of the Navionics Platinum HotMaps product (not available with Premium).
The USA South chip covers about any destination lake I may visit. I like that I will have data for lakes where I won't have local knowledge and could run into trouble fast - even if I don't go there with any frequency. You could think of it as a lower-unit insurance policy for visits to unfamiliar waters!