343 seriously -Yoinked!- up infection.
Infection was never about "oh lets rush the power weapons scattered around the maps and go find the zombies" as it is now, it was about hiding with a pistol and shotgun that had no spare ammo.
The infected were supposed to be hard to run from since they had increased speed etc, now they are a lumbering easy to kill boring mess.
Even halo 4 had better infection and it was horrible.
Oh please, the reach infection was terrible as well. Friendly fire enabled, no boot betrayal, and exploits. Not to mention that the point/infection's system for picking starting zombies was terrible and was purely based on RNG. You could be alpha 3 rounds in a row and end up going negative because of it. Reach infection was broken as hell, giving players the option to boot for legitimate survivor v infected kills. (and even when that was patched, Team killing ran rampant. Hell, I even team killed for quite a while in infected with a party solely just to trigger people.)
"BUT TEAMKILLING IS DISABLED NOW IN INFECTION FOR REACH"
Hahahah no. The fact that it took bungie a FULL YEAR to even address this problem of griefing just shows how much of a copy/paste infection was when transitioning from halo 3 to halo reach. 343i at least came to their senses and stopped the teamkill glitch in halo 4, and the camping issue in halo 5. (not saying 343i is better than Bungie. I loved my time on halo 3 and reach way more than I did with halo 4 and 5, but that doesn't change the fact that the infection gamemode was pretty much left unpolished at the time.)
"BUT YOU TEAMKILLED TOO!"
Your point being? It shouldn't matter if I did it or not, what matters is how the issue was addressed, and it was done near the end of reach's lifetime.
As for "increased speed" Um... No? Both reach and 4 had problems of camping (halo 4 making it worse with the camo). 10% extra speed does next to nothing in Alpha infection, the gamemode that everyone voted for in reach because nobody could have been bothered to deal with zombies using evade, or running to "safe haven". (Back when safe haven was in rotation for infection.)
Also, about your "limited ammo" Claim, specifically in reach's infection... hmm... I wonder if I can get my ammo back by dying to a fusion coil explosion, or a teammate... oh wait.. I can! (In all seriousness, exploits like this negated the problems of running low on ammo, and sometimes even made it easier to survive. Again, this goes back to my point on how exploits ruined reach's infection. (Yes, I know halo 5 infection maps have map exploits, but my point is that reach's exploits weren't addressed until much later on down the line, where it was too late, as most of the damage was already done.)
In short: While I still don't agree with Halo 5's choices for the infection gamemode entirely, they are least addressed some of the issues I had with infection in the past. 343i didn't screw up infection, they just fixed some exploits and changed things to encourage players to run around the map, rather than camp in some dark hallway with only 1 entrance like in sword base.
Edit: After boot betrayal was disabled, team killing hit an all time high. Should have clarified on that. Regardless though, it was still a major problem in infection.