I have seen many large gaps in various places in the world. In some areas the only existing maps are very old and were of questionable sources. In the Amazon River basin the maps are mostly reflecting the contours of the tree tops instead of the terrain underneath. There are so many grid systems, projections and datums worldwide. I usually end up right where the lack of good data is. The introduction of GPS has helped tremendously, but you still don't know what the terrain will be like based on the existing maps in some areas.

Speaking of the magnetic declination, it does change. So the map you may have from 30 or 40 years ago will not reflect the current declination. There are places on line to check current declination for your area.

The differences in datums and grid systems is more relevant when you are sending coordinates to someone else. If you are working on your own map you just need to ensure you know what you are working with and that your GPS agrees with your map. There are ways to convert, but it is a pain.