A tap can disable the current braking completely, resulting in an unexpected change in velocity. Frequently a bad idea, since the ACC is braking for a reason.

Applying normal braking pressure can add your braking pressure to the existing braking pressure, resulting in an abnormally large braking force, sometimes enough to initiate ABS on dry roads. Very disorientating, and it makes you unpredictable to drivers behind you.

The solution is to tap, then press a second later, but this requires you to be aware of the need to take manual control early in the situation.