OSM maps are a work in progress. There will be roads that nobody has put on the map yet, which may in some areas be quite a high proportion. On the other hand, in the areas where they are pretty complete they are better than anything else, having been largely plotted by cyclists with consequent inclusion of cycle tracks, shortcuts etc.
POI files are readily loaded (assuming there is a Mac version of POILoader or that GPSbabel will cope).
POILoader requires that all the POI files are in one folder, and uses the file names as category names on the GPS. Also, loading the POIs discards all previously loaded POIs, so don't discard the files after you've loaded them.
The problem is in getting hold of good, reasonably comprehensive data. If there's an organisation that's prepared the data (Camra, Decathlon, Travelodge etc) it's simple, but there's an awful lot of "beginning" POI files out there (eg lists of campsites with only a couple of hundred sites), and it isn't a simple job to collect and combine them if you care about duplicates.
In the case of the C&CC list, the one I found on the web seemed like it would be reasonably complete, but didn't distinguish between big sites and small member only sites. I use "
Archie's campings" for campsites - the last file I looked at had 21,000 for Europe.