Daemons weren't immune to Force Weapons off Grey Knights before, since Grey Knights force weapons neatly bypassed the Eternal Warrior rule (due to being around before that rule even existed.. or the rule Instant Death), and most of them simply gave +2S rather than the instagib anyway They can get +1S now, but it's only about a 3/4ths chance, with a non-negligible chance of killing your own squad members... and it can be hooded/runed. (Not to mention that Daemonbane is significantly less dangerous than a Force Weapon, given the universal Ld10 that Daemons have)
Also note that tactical marines get free/cheaper special/heavy weapons, which Knights do not (Psilencer excepted, and that only because it forces them to remain still to fire... and replaces both Storm Bolter and NFS). They also have Combat Tactics, which Knights do not, and no real loss of combat potential when taking a transport (and Drop Pods!).
The fact that you're facing tailored lists is simply a sign of poor judgement on your part. I could write a list with most codecies that will trash any other given codex provided I know what I'm facing in advance (and with the imagination of most Daemons players, it's not hard to guess).
As for Truesilver TL-Assault/TL-AC, not only does it show he has little judgement of how to run his dread, it's really not all that. I suppose you complain about how Tau Flechette Dischargers are all OP and stuff to?