I'm a huge fan of the Might and Magic franchise, Heroes included, and that's the only reason I played through this. Unless you are REALLY desperate for your M&M fix, I can't suggest this as a viable purchase. For the same money, you can get much better titles, new and old.
As for the actual problems with the game:
-Graphics are laughable. Expect low res environs throughout the game
-Optimization is bad. I can run Skyrim on ultimate with no issues on my rig, but the turn based M&M is apparently too much, as even on middle of the road settings, there is going to be heavy lag in certain areas with dense foliage. According to the forums, this is a fairly common problem.
-No customization. There are 4 races with 4 hero type each (caster, ranger, tank, melee) and once you pick one of those, you get to decide on male or female. Oh, and also a name. Joy... There's no way to change the appearance of your characters and you never even get to see a model of them beyond a picture during character creation.
-Poor class balance. Rangers used to be completely useless, but after the patch they were upgraded all the way to just "meh". The game LOVES to spring ambushes on you (about 30% of the fights, along with most bosses), by dumping a ton of enemies at melee range, making bows and crossbows useless for the fight (once you engage in melee, you can't retreat till you kill everything in melee range). There goes a quarter of the hero pool. Tanks are also useless, since almost all bosses (and all undead, which are VERY frequent) are immune to taunt - your primary tanking mechanic. This means that 50% of the classes in the game have no purpose, except for flavor, at the cost of your party's strenght. That's just bad design.
-Turn based gameplay on a 2-dimensional plane. You can call this a throwback to the old M&M series, but it just doesn't work. Even the worst of Might and Magic games had a smoother transition between turns, than Legacy. The last ones even managed to mix real-time movement with turn-based combat. Also, turn based movement is not fun when on long treks between towns and quest points. This game has a lot of that, if you're looking for all the hidden little nooks and crannies.
-No voice work for quests. While it's not a necessity for every game, it would have been nice if M&M X managed to show off with something, at least.
-Lackluster story. Everything in M&M X screams "minimal effort" and the story is no different. It's painfully generic and set in a very boring world where every character is just a cookie cutter fantasy stereotype. There's just nothing new, even in the story: elves are elves, dwarves are dwarves and orcs wear fezzes (god damn it, Ubisoft...) You can't even augment your party with new and interesting companions. Once you get out of the hero creation screen, that's it - it's set in stone. You DO have the option of getting two followers, but with no voice work and characterization, all they offer is a boost to one (mostly unimportant) aspect of gameplay.
Relative to what else is out on the market, even at this price, Might and Magic X - Legacy is just a bad game. It has nothing to offer, save for the faint nostalgia of the franchise.