I'd hate to know what you consider a less than astute observation.
The thematic breakdown of MoM is that all races stick to themselves and all sides are led by a powerful wizard trying to become a god and wipe all the other wizards out.
The thematic breakdown of HoMM is a bunch of groupies hanging out based on ideals. You've got the humans and the undead(not that there's an undead side in MoM), but everyone else is way less bigoted. The elves are hanging out with dwarves and sprites, the minotaurs are playing footsie with the centaurs, cyclops is trying to keep his eye on orcs, goblins, trolls, and ogres all at the same time, and those poor halflings really have their work cut out with the magi and titans trying to use them for target practice when they run out of golems. It's all but cheerful until the fifth as well. Even dark, grim storylines built on bloodshed were laced with humor. Did I mention that MoM was grim?
Thematically, the only trait they share is that they're both in a fantasy setting that isn't directly ripped from Tolkien.
Gameplay they're even further apart.
HoMM uses stack based combat. Using the second as an example, you have five slots for stacks of units, a thousand peasants take the same space as a single one. Armies are lead by a hero, hence the naming convention, it's a spin off from the Might and Magic series, rpg's I didn't like. The heroes start out all but worthless, but eventually become game changers, might characters give peasants the offense and defense of dragons, still one hitpoint and damage, but much tougher, magic characters kill the entire stack of peasants, and everthing else but the dragons, with chain lightning. Resources are aquired by fighting guards and capturing pre-placed mines, and the only city advancement is in building a relatively small set of infrastructure. Those cities are pre-existing as well.
MoM doesn't use stack based combat, one unit is one unit. You have a max unit count on any one tile, and that's the biggest army possible. You can obtain heroes, but they are a bonus, not a necessity for an army, and you yourself have a magical presence on the battlefield and main map, as opposed to being a purely imagined figurehead that never acts. You cast your own spells using your own mana pool, summoning armies, enchanting units and destroying the enemy. You aquire resources by building roads to them. Your cities are built by you, and use the tiles around them for your city production levels much like is done in civ. Also much like civ, there is a lot of infrastructure to build in those cities. It's basically a fantasy civ with tactical combat.
In short, they are nothing alike.
