I've been fighting hard for Sarleon since going over to them from Ravernstern around day 100. The Fierds were hammering both kingdoms hard (took Ravernstern town in the first two weeks!) and I wanted to keep fighting them more than anything. I managed to reclaim Avendor and gain it as my own fief, which helped secure the Marshallship for myself. I don't know how the Sarleon lords behaved with an NPC marshall, but I can get most of them (those that aren't already exiled by King Ulric the Expeller) to follow me.
The biggest weakness I've found in Sarleon is their army sizes and composition. The King routinely fields an army above 250 men, which is good and makes the prospect of dethroning him a tough choice. There are maybe two other lords who field armies above 150 men, the rest range from 30 to 100. This itself isn't so bad except that Sarleon seems to get absolutely zero elite mobs.. Among the Fierdsvain it was not uncommon to see Jarls with 30 Beserkers or 20 Omen Seekers, god help me. The Empire has at least two lords who come with 40 Guardian Knights which can just shatter the Sarleon army if enough of them deploy into a battle. Others come with 50 Armored Crossbowmen or two dozen gladiators. I've never seen a Sarleon Lord with equivalent number of top-tier troops. Even King Ulric only brings about 20 Knights of the Lion, tops, with him.
Bottom line is that the Sarleon armies are just too small. They routinely field individually smaller armies each consisting of subpar troops, a lethal combination. The only times I feel really confident is when I'm able to bring 4 or 5 Sarleon lords to bear on an individual Empire Lord, and even that gets messy if he happens to be packing an Elite Mob.
On the plus side, Sarleon has Lady Irina who is my favorite of the ladies so far. Too bad her father got himself exiled two days after I joined the faction.