so we know how much Fear to Tread means for the actual lore.
His 40k books are set in a timeline that cannot happen due to the events that happened with 8th ed.
Fear to Tread is canon, regardless of if you like that fact - the same is true for the entire heresy series.
The flaw is there, but for the majority of the legion, they do not feel its effects during the early/mid crusade. The rare few that succumb to it appear to go full on vampire though - which fits well with the 40k depiction as seen in "Dante".
By the time of 40k it's far more prevalent than it was during the heresy - which matches up with lore we have about the BA gene seed degradation (highlighted as recently as "Dark Imperium").
It has become a defining factor for 40k Blood Angels because like their progenitor, they are obsessed with the flaw, being terrified of succumbing fully to it and determined to cure it.
In terms of it's depiction in "Dante", it frankly didn't seem to have much of a downside for the immediate fight, and only became an issue because of the lasting effects on the Marine, in both instances it kicks in when their life is on the line and they're already in close combat situations.
So, giving it a purely beneficial mechanic doesn't seem so out of place for the red thirst, assuming its a melee only benefit.
If it was campaign play it'd need a complex mechanic where any model that got the bonus from the red thirst (extra attacks on 6s or whatever) also had a chance to sit out the next game or be lost entirely. But in campaign play, we'd also need a mechanic to generate death company too.
"Because I did not see it as glorious Captain Castigon, a bunch of effete over privileged nobles who barely know which direction to point a weapon? Where's the challenge in that? Look at your armour, not a scratch on you is there?"
Blood Angels 3rd Company Wins 25 Draws 5 Losses 13
Heresy Blood Angels: Wins 4 Draws 0 Losses 2
Blood Angels 4th company: Wins 10 Draws 3 Losses 4