While the knights do no seem to be quite as vulnerable to fusion teams as your everyday tank, by the few pictures that we HAVE seen so far, they cost about 50% over a deckout out riptide, give on take. (depend on your "deckout out" setup, and what knight is in question, different knights cost slightly differently)
But essentially, a knight is a "super dread" much like a riptide is a "super crisis", they are both near-impossible to kill in one hit, feature a nifty invul, and have better armor, on top of more wounds (HP is practically wounds), the SH status of the knight also makes them quite more formidable then a regular dread in CC, especially while charging, less likely to end up tarpitted (stomp), being faster, and outright immune to "vehicle damage".
These boys are no laughing matter to put down.
With either dual-shot battle cannon, or a meltagun with triple the range, +1 to S and large blast rules they can also pack a punch themselves in shooting. and there is a third type, yet to be revealed.
Armed to the teeth, not easy to knock down...
Now, how to deal.
With 12 rear they feature, they are immune to any type of pulse weapon, so don't bother there.
The front and sides are even tougher, but not by much. still enough to render missile pods and their likes nigh-useless.
The ion shield is intresting. a 50% invul save, but on once facing only. will help against armies that depend entirely on drop melta, or on ranged bombardment, but as long you got both it wont matter much.
A single knight, is a tough nut to crack, but eventually will be cracked by any army that can properly remove armor.
Pack your melta suits, wake up longstrike-they got work to do.
Riptides can also help, a NOVA shot of the IA has move then a decent chance to put a dent into them-after all, they are tougher then two tanks, but cost more then the same two tanks.
Your main fear, is when they come in groups.
Up to 3 can be as an allied detachment to any imperial army, or if you take that many, and up to double-they can be your main.
Fighting an army composed mostly of them will feel like fighting an armored battle group, except on steriods.
They will cover ground quickly enough, put out impressive firepower, and wreck most forces in CC. you anti-tank weapons will be hard-pressed to take them all out, especially when taking one out puts a serious risk to kill the fusion attackers by themselves.
You will also find many of your guns have been reduced to fancy chopsticks, pulse rifles, flamers, etc cant even get a chance.
Nothing at all. all your horde control guns cant even get the "once in a blue moon" chance to glance, unlike dealing with "just" riptides/wraithknights/etc
Even EMP commandos are unlikely to put much more of a minor chip in the paint, given they will be unlikely to survive long enough to use them in CC.
Also, as they are NOT lords of war-you do not get the fancy escalation bonuses. you need to deal with them the traditional way.
Fusion all the way mates. and hope the dice gods are at your side.