The SF/Inf/Archer thing is mainly just one of cost and utility. SF cost a tonne up front, but are cheap as cheaps in weekly payment. Inf/Archers don't cost that much upfront, but often cost 2-3x as much per week. They do have the advantage in takeovers and civil work though (AFAIK), just due to the number of bodies working.
#normal_troops*0.18*other stuff compared to #SF_troops*0.18*1.75*other stuff sounds pricey for the SF, but it evens out nicely due to having about 1/3rd as many of them.
As an example, I had a unit of 95 archers that would cost me about 130 gold per week for upkeep, rated at around 1200CS at full strength.
I now have (or had) a unit of 33 ranged special forces that costs around 50 gold per week of upkeep, with an estimated strength of 1000-1100 at full strength.
SF's are awesome when on campaign, just due to the time you can keep them paid outside your borders, even on mercenary orders. They are a little faster as well. But when they whiff or get slaughtered, god they cost a lot to get back.
As a side note, those Aestian Arbalests aren't quite as good as I hoped. Of course, I went in with no captain, no banners and a unit cohesion of 24% fresh out of recruitment. Most units will run in that scenario. I really hope they get double-shot at close range, or infantry-archers. I've got an idea that the chance of special ability use is tied to training and unit cohesion, but I could be wrong. There's also some evidence that magic items can have abilities (I had a shield that would rip through undead every once in a while, giving a double-shot with my old normal archers. I really should get that back some time).
