I wouldn't necessarily call give it the "action" title, since you can literally conquer all of Calradia without ever once swinging your weapon if you choose to specialize in leadership and trade. The most accurate description of M&B would be medieval simulation RPG.
Maybe that is a more accurate describtion, like I already said, no one has to take me to serious. I havn't played the game, I just saw some youtube stuff from the multiplayer. All I am judging is M&Bs combat. It seems to have a strong multiplayer going thus I thought that this is what the game was about. I am just saying the Multiplayer alone would not make it some RPG for me. If it has a kind of story where you put your skills to use, things look a lot different of course. Though if it is only about the combat and killing of players, than M&B is not more RPG than lets say ARMA or OFP.
Oh, yes, fair enough, the game does have a multiplayer mode, as well as a pure siege-mode, but out of habit I don't touch those, because they are like... "bonuses"
The actual game is very wide-open, but optimally it is about you, the player, uniting a broken empire, using mostly medieval warfare, but also a wide range of diplomacy.
Again, most role play games can be simplified to this, the vault dweller must also kill a bunch of enemies in order to achieve his goal, and with a very specialized diplomatic ability, you will have less of a need for blood-shed in both examples of games. M&B has less story, being more action-oriented, but it is the detailed character sheet, and the careful levelling that makes it a good rpg in my opinion. If one have ambitions to be a powerful army leader, one must have a lot of patience to go with it, because it's going to take a long time. Killing random barbarians quickly becomes under-paying, in terms of XP, which makes you have to go look for more meaningful tasks (albeit a bit monotonous, since most quests are repeatable random-quests)
But it is much more than just a combat-game. Actually, it isn't so different from Fallout Tactics, come to think of it. You will (ideally) spend a lot of effort refining your team of close entourage (which are separate from the army you ride with)