The Flood were more practically balanced than the Prometheans.
The entirety of Halo 4's difficulty comes from enemies being bullet sponges, you being squishy, artificially making ammo scarce, and subpar level design.
The former will test you, but if you have an itchy trigger finger, you'll manage.
Always keep the Shotgun handy when fighting the Flood. I personally use the Shotgun for Combat Forms, and the AR for Carrier Forms and Infection Pods.
The Prometheans take a while to learn their patterns and make up for their teleportation moves, using the right weapons and catching them with their pants down.
The other problem is the Prometheans, as I said, are bullet sponges and the fact that Forerunner weapons are projectile-based instead of hitscan, yet they behave like they
should be hitscan.
The Flood were rather predictable for me, but they were still fun since they tested your endurance and reaction time.
They both are
and aren't predictable.
The rest of CE teaches you that the different types of Covenant will/won't have certain weapons, and that different colors are harder to kill. Therefore, the Covenant become easy to identify at a distance or within a split second and therefore counter. Zealots go from being bullet-sponge, 1-hit-kill energy sword wielding mini-bosses to being jobbers you can easily dispatch.
The Flood, meanwhile, only have 3 variants (Infection Pods, Combat Forms, and Carrier Forms), with the bulk of their numbers being Infection Pods and Combat Forms, and while Combat Forms might have originally been Human or Sangheili, they are functionally identical in behavior and durability while carrying weapons from everyone else in the game.
So they're no longer easy to identify and counter in the split-seconds you might have...and then you face one that's unarmed, or one that has a "normal" weapon...or one that has a goddamn Shotgun or Rocket Launcher and just sent your ass back to your last checkpoint, but they all look the same from a distance or with only 1 or 2 seconds of a look.
They still have patterns you can learn, yes, but their
strength as an enemy comes from spawning in with random weapons and having zero self preservation, the complete opposite of the Covenant, so the actual flow of a battle can play out differently between playthroughs. And that might be another reason why some don't like The Library...because the Flood don't spawn in more-or-less the same way each time you play a level, with more-or-less the same weapons, so you can't just brute force and eventually learn the pattern to counter and win against.
