Charybdis is so dangerous because she swallows ships whole.
Circe advises Odysseus to avoid Charybdis by sailing along the edges of Scylla’s cliffs because it is better to only lose 6 crew members than all of them and the ship. She also suggests that Odysseus’ men row as fast as they can to make sure that only 6 men are lost and not 12 or more due to Scylla returning.