Fights First reads like a small bonus and behaves like a large one, for a reason worth spelling out.
In a melee exchange, the side that swings second swings with fewer models, because the first side has already removed some of them. Fights First does not add damage to your unit; it subtracts damage from theirs, and it does so at exactly the moment when the subtraction compounds.
The consequence for how you play is counter-intuitive. You do not need to charge. Most melee units only get their advantage by charging, so their opponent can decline the fight or charge first to seize the initiative. A Flawless Blades unit with Fights First is dangerous to charge, which means the enemy's own charge into you is a mistake, and you can profitably stand on an objective and wait.
That is what makes this a genuine Disruption detachment rather than an assault one. A unit that punishes being charged is a unit your opponent has to route around.
The limitation is absolute, though. The rule names FLAWLESS BLADES and nothing else. No stratagem, no enhancement and no part of the rule applies to any other unit in your army. This detachment is worth exactly as much as the number of Flawless Blades you field, and a list with one squad of them is spending a Detachment Point on one squad.