It has some of the best construction rules for new options of any RPG. You get 80% of the flexibility of systems like GUPRS and Hero (although limited here to a subset of fantasy) with about 20% of the work. Not only is there class design but race design, magic system design, magic item design, and spell design. There is also monster design, but I find the value of balanced monster design less compelling than the others.
The author is not afraid to have an opinion. This isn't about politics, but about what is appropriate for D&D style fantasy and what isn't. Thus, the core books omit hobbits/halflings because he's not a fan. When he introduced his dark elves in the Player's Companion they are more Melnibonés than Drow. When he finally gave in and introducing halflings in The Heroic Fantasy Handbook the two classes are bounders and burglars, thus tying them very specifically to Tolkien.
His adaption of the repertoire plus daily casting limit to B/X works quite well as a substitute for traditional memorized spells without straying too far from the base.
The eldritch magic system, his traditional D&D magic plus corruption, is the best B/X magic system out there IMHO, capturing the effects of using various dangerous magics from Tolkien, Howard, CAS, and others very well. In the magic systems supplement it is adapted to cover alchemy (where instead of corruption it is risk of toxicity) and super science (a kind of insanity) showing creativity in reskinning.
It does a lot with economics. Much like I think Lion & Dragon deliver a usable version of the medieval feel Chivalry & Sorcery promised in the late 70s the economics of the pre-industrial world promised by C&S are delivered in playable form here.
Note, for a lot of the above I'm including stuff from all four rules books and all issues of Axioms, the magazine collecting their Patreon items.
If you can't guess, I'm a fan of the system and it is my go to form of D&D these days, slightly edging out OSE and RC/Dark Dungeons. I would recommend grabbing the "ACKS 2nd edition preview" notes from the Patreon, which were published several years ago as document from the future, specifically GenCon 2020. They don't really change much, but clean up some things. You can play with or without them, but they give a feel for where he is going.