Totally agree with opening post. I hate to see that when I play fairly, slowly grinding my player, building economy and somewhat competent army, suddenly half of the map belongs to one faction and then it just stomps over map like it is Golden Horde.
IMHO, main problems are:
1)Recruitment system
Now it's impossible for lord to restore his army after one defeat, he has to roll around the map, getting low quality recruits while winning faction continues to take castles and cities from losing side.
Current recruitment system makes sense from the perspective of a new clan, trying to establish itself, where it has no holding of its own and people, who join its army are desperate enough to risk their lives for a couple of coins, so player has to manually collect them from all over the map.
But it does not make any sense from the perspective of a liege lord, where he has to do the same thing to get t1 troops. Instead he should be able to collect/rally them at the major area holding (castle or city) from his adjacent villages.
2)Troops upgrades
IDK, maybe it is a bug, but t1 recruits even with perk from leadership (which is very hard to attain because of Army system) with medium bonus take too damn long to upgrade to t2. In my latest campaign it takes about two days for 1 recruit to get his upgrade, making replenishing army after lost almost imposible, because instead of fighting invading army you have to chase looters around the map in hope of your recruits getting enough exp to upgrade.
Again, it makes sence from the perspective of a new clan, but:
1)There should be mechanic for player to train his low-tier troops in some sort of camp, which he sets in main map
2)As liege lord you should be able to automatically train your levies during some time and spending some money in your holding at least to t2-3.
3)NPC army focus
In my playthroughs, after initial confrontation in the beginning of the war, when it is almost decided who is going to win the entire war and who is going to be steamrolled, many times allied NPC armies instead of doing something useful like cutting of enemy reinforcement, are rolling around the map, possibly trying to restore their troops, but failing due to point 1 and 2. Some exceptions are when they are chasing invading armies with two-three times larger troop numbers than theirs.
With enough influence(which is also hard to obtain for a player) I think it is possible to collect all the lord to some corner of the map and just sit there, doing nothing, which is ridiculous.
It must be harder to muster large armies, harder to make them staying together.