I'd agree if we were talking about Halo 4, but Halo 5 definitely felt like a Halo game to me.
I finished Halo 5 solo on Legendary within a span of about 3 days, and it was sort of on my first playthrough as well (I did run a few chapters on co-op Heroic first). It was challenging but not that hard. It didn't feel as unfair/unbalanced as Halo 2 on Legendary solo does, in my opinion.
Nearly everything in Halo 5 is a noticeable improvement over Halo 4, and that includes the encounters. The Promethians are more fun to fight this time (way more fun than the Flood ever were), and there were actually a good amount of multi-faction battles (something seriously lacking in Halo 4). The areas also felt more open than the ones in Halo 4 as well, perhaps almost as open as Halo 3's.
But there's definitely room for further improvement. As far as the enemies go, perhaps something like combining the soldier and the crawler to form a single enemy type, one that can change between both forms, and is also capable of sticking to walls again like how the crawlers could in Halo 4. As for the Warden encounters, I didn't mind them too much, but they felt designed specifically for co-op play. The Warden's behavior and attacks should have had more variety, and tied to the number of human co-op players.
The only major flaw in Halo 5 is the team mate AI, which was my only real source of frustration; they were about as intelligent as a used diaper. Giving them power weapons made them a bit less useless "sometimes", and ordering them to focus fire on an enemy was somewhat useful though they'd get themselves killed way too easily. If this mechanic shows up in Halo 6 (and it surely will), the AI seriously needs better pathfinding and better use of cover while reviving team mates, along with a better sense of general self-preservation.
I agree with this.
Though I'd say the one thing Halo 5 was lacking which Halo 4 had was a major space-sim-ish dogfight. I can think of a couple of places where 343i could have fit something like that in.