I've just finished an Expert/High campaign with Scotland and wanted to share my findings:

Scotland in general: Strong on aggressive units, but lacking in long lasting troops, particulalry infantry. Weak cavalry overall, and no mounted archers. Overall, Scotland is a bit of a one trick pony and plays similar to the VI Danes and their Vikings.

Scottish Warriors: Very capable units similar to Viking Huscarles but with a little less defense. These are your best all round heavy infantry and can foot it with CMAA's toe to toe. Big drawback - only available in Scotland which will normally be busy producing the cheap and nasty Val+1 Highland Clansmen instead which are also only available in Scotland. The are available through the whole game though so I'd advise building a couple to start to bolster your main army and be used very carefully with then switching to HC's while you tech up to a good armour upgrades and eventually the Master Swordsmith then spam the SW's with +1 Val.

Gallowglasses: Can be built anywhere with a swordsmith. The Scottish version of the Danes vanilla Vikings. there are a few differences of course, but GG's are impact troops that can cut a hole in anything you point them at. Being able to produce the en-masse allows you to field more truely aggressive infantry than anyone else. It's possible to put so many of these into the field that at least a few will be free to slip around to flanks and tear a pinned unit apart.

Scottish Cavalry: Very cheap at 94 florins and reasonable upkeep Scottish Cavalry makes a good light horse unit for chasing down routers and scattering archers. Lacks the necessary Charge to be a true shock unit but can always go into wedge and spear an engaged enemy in the back. Better hope they break fast though or the Scottish cav will be cut to pieces. down side is that like all Scottish cavalry it is built through the Royal Court instead of Horse Farmer which makes building them difficult and costly. Once you have the necessary infrastructure though, these are a good cheap and cheerful unit.

Scotish Spearmen: Very nice tight unit here. a bit costly, but they have good stats - 1 attack, 2 defense, 3 armour and 3 morale and are produced for the simple spearmakers Workshop making them very accessible.

Clansmen and Bonnachts: Great flanking units, both. The Bonnacts are more expensive, but come with a bigger unit size (100 men) and the bonus of a Javelin each. Clansmen are cheap, come with +1 Valor and can pack a nasty punch as a flanker. Also, being a Fast unit, they can run down archers better than other infantry. Point then at an archer unit, tell them to charge and forget about them. They will chase down and route archers as they try to flee, though they will be exhausted by the end of the chase. think of them as a cheap Archers counter if you have no cavalry.

1. Lack of good cavalry: Aside from RK's, Scottish Lancers are your only real impact cavalry and they don't really cut it compared to other factions.

2. Lack of good all round Infantry: No CMAA's or equivalent unit. FMAA's are as good as you get for an all round melee troop in general. also, no Halbardiers at all, leaves a big hole on your army.

3. No Horse archers: None at all. You can't even get Jinnetes if you conquer spain. The closest you get are Irish Horsemen which are a strictly worse Jinette with less range and no armour piercing but a good charge. Only available from Ireland though and really fail in the mounted archer department. Better build inns and look for Mers if you want HA's.

4. Tough start. Scotland is the only province and it aint rich. A war with the much richer and better armed english is inevitable and tough to win. Once you do though, you can take britain which sets you up well.

5. No crusading though you are Catholic. You get all the cons of being a Catholic without being able to crusade, or build any of the Chivalric units, Feudal Knights, Chiv. Knights or Halb's etc. You sure can get ex-commed though!

General Strategy:

First priority was build up a decent force and drive the English out of Britain. This isn't easy as they control vastly more provinces than you and have more troops. A pitched battle in Mercia or Northumbria decided the issue in my campaign and ulimately spelled the end of the English.

Securing Britain and ireland gives you a number of useful advantages. access Welsh Longbows, plenty of trading provinces to help with cashflow and a very defensible position once you geta navy in the water.

The Scots have one big advanatage over many other factions. They can field a very good army with only keep level provinces. almost all of your units available from keep level provinces - only missile units like Arbalesters need a Castle.

From a Keep province your can create basically your entire army:
Scottish Spearmen - Spearmakers Workshop
Gallowglass - Swordsmith
FMAA - Swordsmith
Royal Knights/Scotish Cavalry - Royal Court
even catapults for siege weapons are available at this level.

While other factions need to tech up to castles and then build further infractructure to get to their better key units like Chiv. Sergeants and CMAA's , Halbardiers and Feudal Knights. Not so the Scots. While other factions are trying to build themselves up they are forced to face you with units weaker than the ones you can field, particularly in the infantry area.

Scottish Spearmen trump feudal Sergeants easily and are on par with Chiv. Serg's but available MUCH faster.
Likewise Being able to field numerous units of FMAA's and Gallowglasses when they are being opposed by Urban Milita/Militia Sergeants and FMAA's.

You have an advantage in all ways except cavalry/horse archers in the early game and it can be a massive weapon for you. Because you are spending less on developing your provinces, you can spend more on bulking up your army and navy nice and fast!

Once you've amassed a large enough force of low tech but high impact forces and Welsh Longbows pick a victim and fall on them with viciousness! The scandanavian provinces are a natural choice, though hammering into Iberia is also viable. the spanish provinces have a lot of Iron, which you lack, are pretty rich and generally quite well developed, giving you a base to build higher quality troops out of and, importantly, those titles of office that can make such a huge difference to a key general.

Also, the only way you can get any Halbard units (other than Merc's) is to take Switzerland. If you do, spam Swiss Halbardiers all day long and pump out the armour upgrades. Without these guys you have to be very wary of enemy cavalry. It will almost certainly be superior to yours and probably more numerous. Your best hope is to rely on your Spears to handle cavalry.

Try teaming up 1 unit of Scottish Spearmen with a unit of Gallowglasses as a 1-2 punch. Engage the cavalry with the spears while you run the Gallowglasses around to the rear and Wedge them in the back. A charge of Gallowglasses in Wedge will cause major havoc and break them pretty fast.

Another thing to remember is that Welsh Longbows - which you will have access to - make great flankers too once they are out of arrows. Including some of these guys in your army effectively gives your 2 units in 1, an armour piercing flanking unit, and great long range armour piercing archer as well.

One final thing to note about playing the Scots - they get a Hero unit in William Wallys - a 5 or 6 command general before any titles are given to him, that pops up reasonably early in the game on one of your infantry units. He is a Great Leader to boot - take care of him and he'll lead you to victory after victory!!

Even against high tech armies like the Crusader States can bring to bear, Scotland can prevail through inteligent use of it's troops. My Army of 9000 scots (Made up of GG's, S. Spears, FMAA's, Clansmen, Welsh Longbows/Arb's, some light cavalry and a few Swiss Halbardiers and Scottish Warriors) has just defeated a Crusader army 19,000 strong packed full of Order Footsoldiers, spectacular heavy cavalry, missile units and heavy infantry galore. At final count I lost 3000 men and they lost 13,000.

They Scot's can have a tough time getting started, but once they get off the mark they can be real giant killers.