No doubt there are some missions that could be better designed, and there are some UI bugs that muck things up as well ...
But when people post their win record with randoms, as if it's some indicator that the difficulty is too high, all I can think is that they've never played firefight before.
Yes, I never played Firefight before. I have been very open from day one that I bought an Xbone to have Halo games for the first time. However, you are still following some idea of a tangible identity of Firefight and what it ought to be, as if you could "get it right" and if you did then it didn't matter how many people didn't like it. That doesn't make any sense at all. People have different skill levels, and as they play and get REQs what they are able to deal with increases. Maybe they continually seek new challenges, maybe they don't. Yet they always want to have fun at their skill level and mood.
Some people would love to go hard at becoming good, but they don't have the spare time. They have a busy job, they have kids, they have whatever else in their life that keeps them from playing as much as is needed to be really good, and yet they still play regularly. For some Halo may be their main game. They can't be obsessed but they are dedicated in their own way. It should treat them well.
Some people are completely casual and would like a casual experience. Campaign is there but after going through it a few times one may want something more. Online is the perfect place to turn except then the difficulty is through the roof. Maybe they play other games where you can get away with being more casual and always being in solo queue and they have no idea why things are so rough with Halo.
Some people go hard and have a regular group of people they can play with often. They are highly skilled and can overcome some really big challenges with planning and organization. They play so often that winning or losing is unimportant to them compared to having their skills properly tested, and winning too often means they aren't being tested, so they aren't experiencing all that they know the experience can be.
Why should some imagined specific definition of "what Halo is" set things against any of these people having the fun they seek from the game? What it "should" be is designed with enough options to satisfy any of them so as to have a large appeal and continue being a very large and popular franchise that can justify its production budget. Playing only to the hardcore crowd will not do that, no matter how much you go whale hunting or begging people to support esports with REQ purchases.
I understand the difficulties of securing quality matches in an MMR system (see: "as a noob I should be able to play people my level, not just get stomped" vs "social is sweaty like ranked, why can't I chill stomping noobs?"), but for a PvE mode? That is very easy to do. Just provide options. Even just one option of a difficulty designed around randoms or around a team.