It seems it very much depends on where you are.
In the South Pacific we have found C-Map to be very good. Navionics is very good in SOME places but definitely not all.
In the Yasawa islands, Fiji, Navionics showed reefs where there aren't any and didn't show some reefs that are there. It also didn't show passages which are marked with buoys that have been there for years.
In New Caledonia however Navionics charts seemed very accurate and the bottom contours particularly useful (is that because NC is French?)
We have both C-Map and Navionics in Time Zero, Navionics also in the cockpit B&G plotter with forward scan (not very useful), C-Map in the Furuno NavNet II at the chart table and the cockpit and OpenCPN with GE2KAP satellite photos on the PC at the chart table and repeater tablet in the cockpit, which is very useful to display along side a chart to see reefs and shoals that sometimes aren't on the charts.
Can't be too careful!