I had an all-infantry army of mostly Vaegir Marksmen for the longest time. They worked well usually, but it was hard to get them to take a good position. I'd ride up to the side of a hill with a good view of the impending battleground, issue the "Hold this position" command, and then watch them all bunch up at the bottom of the hill. Then when the enemy horsemen start arriving, they spend most of the time while the enemy is drawing close backing up and spreading out. In the end they're usually in a pretty good firing position, but they'd be much more effective if they'd assume that position when I tell them to, rather than waiting until after the enemy arrives.
Anyway, the other advantage of the all-infantry army is that they all stay together. My army is now about 50/50 between infantry and horse units, and if I decide to charge the enemy, half of my army arrives first and has to fight alone until the other half finally catches up. But then my tactics skill is so high they usually overwhelm the enemy anyway.