Hi, welcome to the forums. Please understand that there's a lot of flock mentality here. If you ask any kind of legitimate question, expect the first 5 pages of responses to be INFANTRY BAD, CAPTAIN MODE FOR NUBS, THE ONLY REAL CHADS ARE LOLFEINTERS.
I'm not sure this forum is worth anyone's time, to be honest, but here we all are.
I've played almost exclusively Light Inf and Skirmishers since launch. (like ~2500 hours, I'm ashamed to admit).
It is hard.
The historical problem with Skirmishers is complicated. Partly it was the old Scoring system, which did not account for damage done at all, and since javelins rarely kill anyone your Shock Infantry player is gonna be like "LOL I KILLED 100 GUYS REALLY EASILY ITS WEIRD THAT THEY ALL DIED IN ONE HIT BUT IT IS DEFINITELY BECAUSE I AM THE GREATEST OF ALL TIME AND SKIRMHERS AER BAD"
Partly it is because TaleWorlds HATES throwing weapons, because Duel is the only real game mode for real chads, all 8 of the sweaty Euros that make up Taleworld's "Player input" committee. Don't get me ****ing started on what they have done to my throwing axes.
Mostly, it is because Taleworlds hates skirmishers, and the most severe bugs in the game seem to affect Skirmishers the most.
The advantage of a Skirmisher is their hidden "Combat Move Speed" bonus compared to other troops. This is not communicated to the player in any way, but they can accelerate multiples faster than other troops when in the "Combat" state. Only your Captain can really take advantage of this, and individual AI will foolishly decide to engage sometimes when you've told them to move to another position away from a Heavy Infantry troop they could easily just outrun.
The design intent is to use this with the Fallback command, harassing enemies and stringing out their formation so allies can hit them with a charge or arrows.
Skirmisher and Horse Archer AI is currently bugged as of the last patch. They seem to confuse the direction of their intended move target and the direction of their nearest enemy very often, and you'll watch them throw stuff at walls if using the Fallback command which is absolutely vital to playing Skirmishers. ( I have reported this, as I am one of 2 Skirmisher Main players that probably exist lol)
Skirmishers are only viable on the the very large maps, and only with certain team compositions against certain enemy compositions. You need either friendly archers or horses to take advantage of your ability to push and pull enemy units around the map. If the rest of your allies are heavy infantry, you're relegated to trying to hit the flanks of formations with your javelins. If your enemy is primary heavy infantry, you wont affect the battle too much and have to do support charges with your allies, or just be a nuisance and attempt to goad enemies into chasing you.
Couple of Faction/Equipment tips:
-Only use the Spatha on Recruits. The AI doesn't understand that the Pugio is fast and does not attack any more frequently than normal with it, and the Gladius is not a weapon at all. I wouldn't ever recommend using an Improved Shield perk on any class in Captain as shields break so infrequently it is really a non issue, and the AI is able to pick up new shields.
-Don't use Throwing Spears as throwing weapons. Their damage is nerfed to the point of absurdity, you'll regularly do 20-30 damage on Rangers or Peasants 5 meters away (literally the same damage as the lightest javelins). They also cannot be picked up by the AI once thrown. Their only use case is if you want to screen friendly archers against cavalry, which Skirmishers are surprisingly good at with their very fast combat move speed.
-Rely on the Rapid Throws perk. There are instances in which the Better Javelin/Specialist perk is good (If your enemy is spamming Shock infantry in a smallish map), but with such poor accuracy and limited ammunition it is a roll of the dice if you'll have any effect in combat. The max damage output of the Rapid Throws perk is much greater, but will take a longer time to output.
-Always take Kaskara on Skirmishers. The Flyssa is basically not a weapon. If you beat up a Shock unit with javelins and then rush them with the Kaskara, you can often wipe a Menav or Hammer Savage unit on your own, which is really unusual for Skirmishers in general.
-Aserai Skirmishers are unusual because they can have a dedicated spear and a large shield with javelins, meaning their actually good at protecting your archers from enemy archers and cavalry, and can even do a volley of javs when enemy shock infantry tries to charge your archers.
-Brigands are the absolute worst Skirmisher overall. Their troop count is very low, their equipment is some of the worst, and they are not very fast. The only reason to use them is to have a Spear and Shield with a only very slightly higher troop count and move speed than the Varyag. Can sometimes be usuable if your enemy is focusing on using horse archers in a map with tight quarters, but nobody does that.
-Wildlings are weird because they are basically Medium Infantry with Javelins. If there are enemy heavy infantry on teh field, take the Club. Use your mobility to catch opponents in bad positions. People often get confused and think their shield is good for shieldwalling, but it has a weird hitbox and you will be torn apart by sustained archer fire. Their real strength is staying a few meters behind Oaths or Rangers/Fianns and discouraging enemy charges with their volleys and immediately joining the fight with Clubs against heavy infantry.
You can't engage enemy heavy infantry alone, can only engage enemy light infantry after throwing all your **** at them.
Taking them is a huge gamble, but it can be fun.