VNAV is a finicky beast. In general operations, we use VNAV PATH, as it computes the most efficient descent path based on an idle thrust descent. If the winds in the FMC are wrong, it will allow the airspeed to fluctuate around target speed slightly, before reverting to MCP SPD mode to increase thrust.
Unfortunately, it has no control of overspeeds. Target speed + 10kts leads to a scratchpad message (on the 737NG at least) of DRAG REQUIRED, and its up to you to manage the speed. Ignoring this too long can sometimes lead to the VNAV disconnecting.
Using the Speed Intervention switch will revert to VNAV SPD, where the aircraft will fly a target speed, and in some cases disregard the path. For instance, when in VNAV PATH, if you are above target speed and there is a speed restriction approaching, it gives the path priority.
This can be dangerous too, as VNAV has no VMO or MMO protection built in. If you are 3000 feet above path at say 320 knots, it will pitch for the path, and the airspeed will rapidly increase, and exceed max operating limits.
Horgy