I don’t think it’s just a leadership issue: it’s almost all across the board.
Some abilities level up decent enough because they are overtuned. So a horse archer hits a sad soul in the head at max range flock shooting into a crowd of 200 men and gets 15 levels in archery and 10 in riding. Or someone kills 3 people with a siege weapon and now they have 3 perks unlocked. This love has always needed to be shared around.
All Skills need primary, secondary, and tertiary ways to gain XP for them. In addition, all missions you complete should 25 bonus XP, divided up in 5 groups of 5 (which can all go to the same category, such as the art of trade giving you 25 trade XP) and that should then apply to all tag along characters giving them 15 bonus XP, divided up into 5 groups of 3.
Leading formations as a captain should give tactics XP. Upgrading troops of the type your companions fall under should give them some nominal leadership XP (even if just 1 before the multiplier). The first time you play a board game that day, you should get 10 tactics XP if you win, 5 if you lose.
Etc etc etc.
Having layers ways to gain all control in all areas allows for many more ways to approach the game
I can understand them making it easy to level up things like horseriding since damage on horseback is all down to speed. A slow horse will get you killed very easy. Scouting too, I understand why it's easy to level up, there aren't really any major benefits from leveling it up, aside from being able to catch wild Wader Hotbloods. The leveling balance for the player skills is perfectly fine, it works.
It's the companions skills that are the problem. Their combat skills level up fine when they are with you but to level up their leadership and tactics they have to be made leaders of their own party and that is very expensive to do, especially in the early stages, but if you don't level them up in the early stages they become easy marks when your ready to found your own kingdom and you end up constantly running around cleaning up after them because they are so easy to defeat dispite their combat capabilities because their tactics and leadership skills aren't high enough.
I like to play as a Battanian, but battalions develop towns slower than most and have Vlandia, Sturgia and The Western Empire on their door step. So every time I play as a Battanian Hero, I level up to clan Tier 4, set up maximum companion parties and declare war on Caladog. I like to time it so they are in trouble when I pick a fight. I take one of their castles and play Zone Defence to level up my clan. The problem is while I'm doing this the AI is getting loads of leadership and tactics exp, but my companions aren't. So when the time comes to promote them to clan leaders, all the hard work ends up in vain because they don't have enough leadership and tactics experience to go with their combat capabilities. I can conquer Battanian as a obe clan Army easy enough, keeping hold of it with only my companions clans to back me up is another matter.