The Heavy swords light attacks (the Vanguard ones that do the most damage) are too slow to come out, slow you down too much, and deal too little damage to compensate for this.
Their damage is barely higher than the Catachan sword’s lights. There’s a good reason why a lot of people like using the Mk9 : for horde clear the heavy sweeps feel much better.
In terms of moveset, the light combos are also really boring. The differences between the ‘Vanguard’ and the ‘Relentless’ swings are so minimal they can be ignored.
The low finesse bonus of the lights (especially the ‘Relentless’ attacks) does not incentivize aiming for headshots at all, because it kinda does f**k-all when you do hit heads.
The special attack is fun to use because you keep your normal mobility all throughout the attack, but its low cleave, and the fact that the light follow-up is bugged and very weak, are kind of a bummer.