Sea-tiles are colored the same as land, so any contours of landmass will eventually render the minimap useless. CIV5 had the same problem with the endgame replay - you would end up with a couple of big colored blobs. If you played an earth map, you wouldn't recognize the map in the replay. If this bleeds over to the minimap in CIV6, then it's basicly useless.
This how CIV5 does it:
This is what CIV6 might look like if nothing is changed:


This how CIV5 does it:

This is what CIV6 might look like if nothing is changed:
