The FCR 1.6 manual has this section:
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Strong - used mainly in fighting unarmoured opponents. It is useful for
fighting monsters where a lot of strength is needed to pierce their tough
skin and mounds of flesh.
Fast - used in fighting armoured opponents. When the body is protected by
steel, scales or chitin armour, strength does not matter anymore. The fast
style consists of quick, precise hits aimed at unprotected spots.
This makes me think the mod isn't properly thought out. Granted the vanilla game was weird compared to the documentation (you use Strong style against half naked thugs?), but this FCR explanation for when to use Strong vs. Fast is completely counter to how fighters fight historically. If this is a reasonable way of fighting, then knights would have been carrying rapiers and swashbucklers would have been carrying claymores. It is my understanding that historically, knights switched to two-handed weapons because they needed that extra power to penetrate increasingly heavy armor until widespread use of gunpowder rendered armor obsolete.
When you fight armored opponents (whether their armor is plate armor or thick hide), you need armor-piercing or concussion attacks--Meaning half-swording techniques, powerful crossbows, military picks, maces, flails, etc. That lets you score some sort of harmful effect past their armour. And you use a specialized weapon when the enemy is helpless and completely unable to defend, like a misericord through the eyeslit of a helmet.
When you fight relatively unarmored opponents, you use a light weapon and fast technique not because heavy weapons were obsolete but because it is more efficient to score a critical effect like a thrust to the heart or neck. That results in a faster kill or at least a fatal wound. This is why duelists and swashbucklers used rapiers and foils. It did NOT stop them from wearing armor to protect critical locations because armor still works. But armor couldn't reliably stop a gunshot so armor fell out of vogue.
What vanilla Witcher did with styles was not really about armored or unarmored, Strong or Fast (they are just crude names for the two styles). It was about the enemy combat style relative to you. When the enemy was very mobile, it is harder to hit them with your slower but more powerful swings of "Strong" style. If you could handle it (i.e., skilled because you developed your Attack rating), then you could still land hits and you could inflict heavy damage.
When the enemy had no finesse and just came at you (big thugs with axes that relied on brute force rather than technique), or wasn't very mobile (e.g., echinops), you could use Strong style more easily.
You used Fast style to counter mobile/agile opponents because you had to, but the trade-off was less damage. Why you couldn't use Fast style on stronger opponents... that is a mystery. Presumably they had more toughness to shrug off your lighter blows, but the whole Strong vs. Fast distinction collapses when you can somehow hit everyone when they gang up on you with a mysterious Group style that ignored the need to choose between Strong and Fast.
More probably it was a mechanics issue, done so that you had to spend Talents across the six styles. Which is not a great way to handle it, but the number of Talent points you get versus the gameplay experience you were supposed to have being Geralt of Rivia is a different discussion altogether.
If I were to overhaul fighting, I would make Group styles a modifier that increased the arc of your swings. Assign Pain (staggering effect), Knockdown, and armor-reduction to Strong Styles; assign Bleeding, Disarm, and Precise Hit to Fast styles. Fast Styles do less damage (and so are more affected by armor) but have more blows per unit time. Strong Styles have fewer blows but compensate with armor reduction. No severe penalty to hit for using the "wrong" style against an opponent.