Having bought and played through the whole campaign as well as a bit in Skirmish and Sandbox, I can say that there are a lot more problems beneath the surface of bugs, performance issues, and bad AI. The game and level design are both overall quite poor or at least significantly inferior to that of either Dk1 / DK2. It's an overall fairly shallow experience because both the dungeon management and combat side of things have been greatly crippled.
Simply put, there is little to no actual management that needs to take place as everything is automatically done, including critical things like converting in the Torture Chamber. There is no danger of making a mistake because of the ability to rebuild walls and gain infinite gold through one of three methods. Mana is capped low, instead regenerating very quickly, eliminating any real need to watch carefully how much mana you spend or have left. There's even a potion that grants experience, as well as many other ways to expand your army without even leaving the dungeon.
Combat is a complete mess and it's virtually impossible to tell what is going on or who is even winning sometimes. Creatures can clutter up on the same tile so it's sometimes not even possible to tell how many units each army has left. Due to the inability to pick up creatures off your own land, the balance between defensive and offensive play has been completely skewed in favor of the defensive player; this is to say nothing of all the new ways to gain an edge while fighting defensively that are denied offensively. Competitive multiplayer will never be able to exist because of how dull it will end up being as a result of this very obvious imbalance.
There's a lot of new features, sure, but there's also a great deal of fluff. There are many things that do exactly the same thing. There's a Construct (think offensive trap) that destroys walls and a potion that does the same thing. There's a Volcanic Bridge spell which builds a bridge over water or lava, just like the Wooden or Stone Bridge. There's even a spell to mine a wall or claim a tile, but it's restricted to land adjacent to yours and it can't be cast on enemy land; it's basically a more expensive and limited form of Create Imp. The list goes on. Defenses (Traps / Doors) are all basically useless because of the double means to tear down walls and completely bypass them unless they're built around the Dungeon Heart.
The Campaign is just outright terrible. There's a short 13 levels that litterally look like they were made in a week or two. There's no real pacing and almost all forms of difficulty are artificial, where you're set up to face off against an overwhelming force but the solution is to just sit around in your dungeon and grind up your army for at least half an hour. The levels themselves don't even look pretty, instead appearing to be incredibly uninspired; level 10 has a massive hero fort that appears to be three large room designs copied and pasted all over it.
The plot is simple and predictable while also being poorly presented. Characters and vitally important artifacts just pop up out of nowhere for convenience. The player randomly speaks at two points in the campaign, which is confusing in and of itself as it's not obvious who is speaking. Aside from Richard Riding's voice, voice acting ranges between "acceptable" to just outright awful. Richard Ridings only makes it much more apparent by comparison. The writing only makes things worse, appearing to have been done by a teenager; this goes for both plot and random mentor messages you get every 5 to 10 minutes.
Overall the game just has no charm or any real redeeming factor. It's completely riddled with poor game design choices, which has absolutely destroyed any concept of balance while also making the game basically a watered down version of either DK1 or DK2. It's just a massive disappointment.
tl;dr:
Game sucks, even if you remove all the bugs. Do not recommend.