The mod has many changes made to it,including some encounters with enemies(that change was not made by me,it was by Ruby of Blue,i'll cover what he changed).
Mostly what is needed to be said is the fact that there will be differences between the first and second half of the game.In the first half changes to ammo and firing to enemies has been made in a way where enemies will not be totally incompetent,especially on legendary.However in the otherl half of the game you will see that lower ranking units like jackals and grunts after witnessing what the flood can and will do to them they kinda went crazy,firing their weapons agressively,faster and more brutally so staying out of their way is necesary since they dont care anymore to preserve their ammo.Also you will quickly realize that your ammo goes out quick,that is because enemies barely have any ammo after being exposed to over 12 hours of constant fighting the flood their numbers also dwindling so did their ammo,so as you the player have to be more careful with how your shooting the weapons,firing to much will leave you ammoless(basically defensless).And if that wasn't bad enough UNSC weapons are absolutely useless on the flood(from flood lore,if you read it,the flood dont use a bodies nervous system to keep it moving so shooting it with bullets will result in nothing)the flood in halo are like the necromorphs in dead space,shooting their bodies with bullets does next to nothing,however the plasma rifle,needler impact and the shotgun are absolute shredders for them,a flood elite with shields on shot by a shotgun point blanck equalls death.Explosives are also highly recommended for use against the flood,flood elites have an advantage with the fact that plasma grenades are no longer the powerful destructive grenades they once were.They are mostly EMP grenades,however the same cannot be said about the UNSC's powerful frag grenade.That grenade kills even a shielded flood elite.
There are obviously changes to encounters and the way the game looks like(kinda like its xbox counterpart,not totally but its pretty close=these changes are made by Ruby of Blue with the encounter changes,and the refined version of CE made by refined halo team)
Weapons underwent many changes to make them more realistic,also covenant weapons lost their autoaim and magnetism while UNSC weapons lost only autoaim:
     UNSC
     the AR(assault rifle)is now muuuuuuuuuuuch more accurate and is more effective on elite shields but useless on hunter armor
     the magnum is perfectly accurate,as all headshot weapons are(making these weapons depending on how accurate the player is)its more effiecient on shields and is muuuch more efficient in taking out sentinels as vanilla CE it was barely doing 1 fifth of its actual damage,also the weapon is now semi automatic but it fires as fast as you can pull the trigger
     the sniper is an absolute beast capable of killing pretty much everything aside hunters and flood in one hit.Shields obviously protect elites,but minors and majors go down with one headshot
     the shotgun is twice as accurate,not as useful on the player to keep it from one shoting you,does half the damage but double the pellets it fires.meaning that its more useful on flood,and since its effectiveness has been increased by 50% on elites shields,it almost kills an elite zealot if fired from point blanck range
     the rocket launcher,lets just say it went through sooooooooooooooooome miiiiiiiiiiiiiiiiiiiiiiiiiiiiiinor changes.Im kidding it went through most actually,its explosive power kills everything in one explosion aside hunters and tanks,not even zealots can survive the bare minimum damage the rockets explosion does.and it also has esplosion radius increased by 20%
    COVENANT
    the plasma pistol is no longer the monster it once was,its pretty bad when used on flood,hunters and elites but still very strong against jackals and grunts
    the plasma rifle has been tweeked thus its not 4 shoting your shields but like 5 to 6 shots,with the fact that now the flood have a bit more health the plasma rifle needs like 7 shots to put them down
    the fuel rod gun is almost as useful as the rocket launcher(there is no difference between the hunters fuel rod,grunts fuel rod and banshees fuel rod gun)its capable of killing unshielded enemies with one explosion,impact does extra damage and kills major elites,hunters can go down from 2 to 3 shots from this thing(unforcunately I dont know how to make the fuel rod gun usable in halo CE so you cant actually use it,sorry)
    VEHICLES
the warthog's chaingun is perfectly accurate and is more useful on infantry however its pretty useless on wraiths,ghost and banshees main gun are also very useful on infantry but pretty useless on wraiths(effective are explosives on the wraith)the scorpion tanks muzzle is perfectly accurate and faaaaaar more powerful,still 2 shots wraiths but everything else will die with one hit,even hunters the scorpions chaingun is like the warthogs chaingun,perfectly accurate and its rate of fire is increased.Vehicles explosion damage has been increased such that they cause certain allies and enemies to die from its explosion,to make it more real,i mean when you watch how a wraiths explodes in a huge fireball and the ai just 2ft from it is untouched,yaaaaaa
    AI
   Your allies have been changes as they were pretty incompetent on the battlefield,being stunlocked with each shot and barely firing their weapons,now they are less stunlocked,not with much(change by Ruby of Blue)and they fire their weapons still in bursts but the separation is barely 0.1 to 0.2 seconds.allies fire the and sniper faaaar less frequently since they would easily overpower enemies(its to show how the sniper rifle fires anti artilery rounds it has a lot of recoil)so their firing rate is drastically decreased but they are still very accurate and dangerous(allies are pretty weak to UNSC weapons and die from everything that causes explosion aside the sentinels explosion)
   Enemies are also more competent,pretty accurate but elites no longer track you perfectly,and lead to enemies has been decreased by a bit,it felt off,it made them math masters calculating pretty much perfectly how they have to fire their weapons to hit you,certain enemies have more health while others have less,grunts and jackals have less health and hunters and flood have more,dont worry they are still pretty easy with explosives and shotgun to hunters even the sniper.BTW hunters are no longer one hit kill from pistol and sniper.

BTW to play the game,simply extract the file using 7zip,place the extracted maps into your Halo Custom Edition maps folder,warning this doesnt work for Halo Annyversary.