One problem with using Chainmail's man-to-man combat in D&D is that many monsters don't use manufactured weapons and armor. For those that do -- orcs, goblins, ogres, and so on -- it's easy enough to use. For those that don't, we either need to assign their natural weapons as a weapon class or else come up with some numbers just for them. And I don't think a generic "claw" or "bite" attack line makes much sense when you have everything from giant rats to dragons using them. So each monster would have to be evaluated as to what weapon is closest to its natural attacks, and how they compare against different types of armor.
Then again, if we're still using weapon damage, then maybe it's fine to have one "claw" attack line or what have you. My friends and I mistakenly used the 1st level hit roll numbers for all monsters and even for higher level PCs for the first few years we played, since they were printed on the character sheet on the back of the Mentzer book. So dragons and giants and rocs had the same hit probability as those giant rats in our early games. Dragons and rocs just did a LOT more damage when they hit with their claws and bites. Maybe having numbers for any "claw" or "bite" or "tail slap" or whatever would work. I'd still need to assign those numbers vs each armor type, though. Or decide that all tail slaps count as morning stars and all claws as daggers, something like that.
And then we turn to armor. D&D of course abstracts thick hides, quick movement, large or small size, etc. as part of a generic AC, while Chainmail man-to-man specifies the type of manufactured armor worn by an opponent. AD&D of course kept the weapon vs armor table which is based on Chainmail man-to-man (I assume, never checked the numbers to see if they more or less match). It's one of the things I never liked about AD&D and never used when I ran it, so I don't remember if it's just hand-waved for creatures with a certain AC but not assumed to be armored, or just ignored. For this system I'm developing, though, I can't really ignore it if man-to-man combat is going to be a big part of the game.
Alternately, when fighting animals, bestial monsters, etc. we only use the mass combat rules, or Fantasy Combat if the creature is on the list (or equivalent to something on the list).
Of course, if I do simplify the man-to-man tables to match the mass combat armor types instead of the detailed breakdown given in Chainmail, that might make it more manageable. But it's making me think more and more that the system in the Dungeon! board game might be simpler than Chainmail's system. Especially if monsters are just given a general chance to hit. While Dungeon! gives the same attack roll for all monsters, I could give some variety so that bigger, faster, or just more dangerous monsters hit more easily. But then it would negate the bonus that Fighters and Clerics get of wearing the best armors. So I'd need either numbers for armor types, or numbers vs class (the way Dungeon! gives each class different numbers vs monster type).
Or, to make a long blog post short, I understand why the "alternative" combat system using a d20 vs AC became the standard. Many fewer headaches. I'm not quite ready to ditch Chainmail, though, as I think it might make combat interesting.