I think Halo 5 was the first real game I got into that had "loot boxes", so at that time it didn't bother me as much: they barely do even now. However, there's problems with the system that I can't ignore, and that need to be changed.
Anyway, here's some pros and cons to the REQ system (there's prolly more):
1. Obtaining rare items is far easier
This is a debatable Pro. Setting money aside (because we know that usually always makes things easier), if you grind enough you'll be able to get the sufficient number of packs to get what you really want. I don't know the logistics of it, so I'm not sure exactly how long you have to grind, but I believe that it's not as bad as having to grind for armor in Halo: Reach. Almost everything is up for grabs, unlike Reach where most of the armor is locked behind a cR wall that everyone has to overtake.
2. REQs can be gifted
There's nothing like this in older Halo games; the ability to gift someone armor, weapons, what have you. I mean, you could transfer an Xbox profile to someone, but that's like, really antiquate.
3. Special, non-REQ armor still exists
Achilles, Olive,Timmy, and some 343 Fotus and Assault Rifle skins I think are the only ones, but that's still stuff you can't spend your way to get.
1 It's Pay-To-Win in Warzone
The "assistance" may be marginal, but having access to the most powerful weapons in Warzone because you have money - even if you don't utilize them correctly - makes it Pay-To-Win. Honestly, it's Warzone, so I didn't really care to begin with: Arena is still your precious, core Halo experience. If you've got enough skill you should edge out the rich kids, though the odds are stacked against you.
2. REQ drops are random (except in certain special REQs)
There's a higher guarantee you'll get something important with Legendaries and whatever, but you know, it's still randomized gambling.
3. Most REQ items are decreased in value: Quality over Quantity
In order to make REQs work as a genuine Loot Box experience, 343 had to make a ****load of "filler" to be contained within. Most people agree that Seeker is a bad set, but I don't know what the hell they're on about: it looks FAR better than Pioneer, which nobody had **** to say about back in Halo 4. I don't agree that 343 didn't put any effort into most of the sets, since allot of them I really like, but I have to agree that having so much of it devalues the overall group. Not to mention, if you can get the most rare stuff by just paying for it, it undermines the achievement of getting that through hard work and dedication.
3. Loot Boxes prey upon the younger generation: Halo 5 is Rated T
Laws against minors gambling, bla bla bla, we all know what's going on with that. Multiple countries are seeking ways to regulate gambing to prevent Loot Boxes prominence, and REQs I think fall under the Loot Box umbrella.
So, how would you fix that? Well the obvious solution is to not have them at all, but I don't think they'll go away just like that. Still, my ideal solution is this:
Destroy REQs. The armor and weapon system returns to that of Halo: Reach where you earn credits necessary to buy stuff. However... there's an option to spend money for these sets, and in some cases, there will be big armor sets containing exact pieces. There are benefits to either buying a set, or unlocking it in-game: such as being given different variations of that same armor to ensure both camps feel special for having unlocked it with money or with credits. You would also make it so that either of these alterations cannot be achieved by the other camp, making that exclusive.