Hi folks, I just wanted to write a quick breakdown of each army and how they can destroy Imperial Knights.. seriously no need to get scared
Imperial Armies: Melta, melta ,melta
Chaos Marines: Melta, daemon engines
Daemons: Greater daemons have superior CC stats then Knights, you'll have to use superior speed to gang up and dogpile each knight, one at a time
Tyranids: T-Fex rupture cannon, hive guard cannons, zoanthropes, crone, dog piling monstrous creatures lol
Eldar: Bright lances, heavy wraith cannon, fire dragons
Dark Eldar: Dark Lances, Void lances, Wyches haywire
Tau: Fusion guns, Railguns
Necron: Guass
Orks: I'm not familiar with Orks, but their rumored to get a new codex very soon
Imperial Knights in my opinion make great support units, but a pure Knight army will suffer diminishing returns. Don't forgot that Imperial Knights can't in any way, shape, or form.. hurt Fliers!!! Blast weapons can't target fliers at all, so pure knight armies might be impossible to win with. Also flying monstrous creatures never have to land, they can just spam vector strike the Knights until they blow up! Rock, paper, scissors baby! Hope this was helpful