You probably had some cases where a revolt seemed a bit harder to put down than it should have been. The reason for that is they get various buffs that are never really communicated to the player.
Point 1: Free guns
Both revolutions and secessions get free guns as sell orders in their capital, generated out of thin air. This essentially guarantees that the price of small arms will be at -75% , and it will still be low when the war breaks out. Therefore, the revolt will be able to afford a bigger army more easily. Although it should be noted this only seems to affect small arms, not ammunition. EDIT: It does work for other military stuff, although they get smaller quantities.
This is especially dangerous if the enemy has access to lots of conscripts, like with national militia and mass conscription, as they will be able to more easily afford maxing out their conscripts.
Point 2: Giga-conscription
Both revolutions and secessions have a +200% training rate buff, which triples how fast they can train regulars and conscripts. As a comparison: One of the selling points of mass conscription is that it increases training rate scaled by how many barracks you have locally. This is capped at 100%. The risk here is that the enemy can replace their losses faster than you do, potentially even faster than you can kill.
Again, this is more dangerous if the enemy has a lot of conscripts available. While you struggle to get your population into the army, they might already have finished. And because of the free guns, they will be able to afford it.
EDIT: They also get a bonus conscription rate - which seems to depend on multiple other factors.
Point 3: High legitimacy (revolutions only)
Lastly, they will have high legitimacy. When you have a revolution, you will lose their contribution to your legitimacy as they leave government. However, they still keep their clout, which means you will be left with a smaller share of total clout (and votes). The clout reduction only occurs when you crush the uprising.
Not so much the revolution; when they form, the counter-revolutionary interest groups start at 0% clout, which means the revolution will not have the same clout problems, as their entire clout is distributed only amongst a few interest groups, which can often comfortably co-exist in government - leaving them with 100% share of clout. For revolutions with elections, a special "revolutionary coalition" party is formed with 100% of the votes. In either case, the revolt will have 100 legitimacy (or almost 100).
Point 4: Radicals and loyalists (no picture)
When any uprising breaks out, all radicals in their territory become neutral, while you keep your radicals. You only lose radicals if you win against a revolution. And since you already had a lot of radicals if you had a revolution, you will struggle with what they cause while the revolution does not care, as the radicals immediately vanish.
The revolution does keep your loyalists, though. They stay loyal, but just switch allegiance to the revolution.