This is a reflection of the two modes in comparison with each other. I played both recently to refresh my memory and I still think ODST was the best iteration.
For some context, I notice some denote a mythical praise towards Halo: Reach, especially its Firefight mode. For me though, and all those I knew and played with (not to mention those old Bungie threads with a nice chunk of the community as well), I still consider ODST to be the best iteration of Halo’s ‘third mode.’ Halo always had single player and PVP multiplayer, but when they first introduced ODST Firefight, the first PVE in Halo, the vast majority enjoyed it. It was a great mode that emphasized survival and consistently increased the challenge, but in unique ways (not though just buffing enemy health). Each map was very distinct and some had a night time variant. As a consequence, killing enemies would need different methods over time. The rate that the methods would change was also nice because it allowed you time to adjust and get comfortable, but not enough to get bored. Overall, I considered it a great first iteration. This wasn’t true with Reach.
The mode came with a large variety of settings to customize Firefight, and that was great, but the entire tone and purpose of the mode had shifted away from survival. The introduction of the unlimited weapon cache was the first major cause of this. The options were have unlimited ammo for base weapons (minus grenades) or to have no weapon cache at all. This removed the ability to recreate the survival aspect and pushed a certain playstyle which contrasted greatly with ODST’s Firefight. The next were the AA (especially the Jetpack) addition, albeit optional in name, made all maps designed for them in a poor manner. Maps were all exploited the same way because of this (this, meaning AA and weapon cache options) and the consequence was they all played the same. The byproduct of this also affected player strategy for killing enemies despite the good variety (really poor AI diversity amongst enemies actually) as none of them would get close to you for. Finally, skulls had next to minimal effect on strategy or enemy health, again due to the maps and AI. This meant all enemies were killed the same way for the entire game. Overall, the consequence was a grindy and repetitive action mode. The survival aspect was really in name only.
Ultimately, I still have yet to see them recreate a survival mode. Another related observation is that Halo 5 took an entire left turn, and stopped pretending it was about survival like Reach did, and went to an objective based mode in Warzone. Kudos for that I guess, but I think they could have done that mode better just by adjusting the lives and timers.
I have no doubt Halo Infinite will have a third mode, and past evidence shows it will be PVE again. The question is though if they will make a PVE survival, objective, or action. I hope they bring back a survival game mode like ODST firefight rather than an action game mode like Reach firefight.