Wing tanks are relatively wide but not very high. This means that if the aircraft is a few degrees off-level (on the ground) or not quite in balance (in the air), then this will lead to a relatively large level change at the point of the fuel level measurement probe. This makes them relatively inaccurate.
With a fuselage tank, you'll find that it is not as wide for its height, making fuel probes somewhat more accurate and less error prone due to being out of balance.
But it's never a good idea to hop in a strange aircraft and then fly it to the limits of its theoretical endurance straight away. Be conservative.