I just spent hours on High Charity on Legendary, with Juggernaut modded in, and I can easily say that Halo 2 Flood are the worst Flood ever. Shotgun is near useless against Flood; swinging the sword wildly is pretty much the only reliable way of dispatching nearby ones; bad hit detection on the human combat forms; Elite combat forms are randomly shielded; Flood spawn in front and behind you; a speedrun in which you must kill all the enemies is extremely luck dependent since battles between Flood and Brutes don't always have the same result. Halo 2's high learning curve really killed the fun. There was only one exception: the Juggernauts. 1 hit kill or not, they are really great to have around.