[Mossy Toes]

Can't everything you listed there also take out a Land Raider at range, flat tiring your advance?

It's possible a Wraithknight or Knight could get lucky with the right shooting weapons, but they'd have to assault the Land Raider to really have a good chance at crushing it... in which case, if they're blocking off your lone exit point, ouch that's an emergency disembarkation, but if they're not... you just pile out and lay into them. A Riptide wouldn't do that well against a Land Raider unless it was that new super-Haywire experimental rules FW Riptide. And even that Riptide would have to get within 12" to do so...

 

[Mossy Toes]

For some reason I thought all those models had more long range anti armour than that.

Well, an 18" melta-range large blast on the Knight isn't bad, but still has only a 13% chance or so to kill a Land Raider outright (assuming in melta range and no cover save), assuming you're taking that build of a Knight. And 2 S10 shots from a Wraithknight ain't half bad either, if you're taking the D-cannon weapon, whatever it's called, there. Still, they're not reliably going to take out a 4 HP, AV14 vehicle...

 

[Mossy Toes]

I'm assuming that you can move at least 6" with a Knight and still fire that weapon which would really give it a 24" melta reach in your own turn,

12"--it's a walker that can move that far every turn and still fire its 2 guns.

The 13% number I crunched a while back is for BS4 regular melta having a 2/3 change to hit, true. Crunching the numbers of a large blast is... fuzzy, at best. But true, you'd have to roll about a 10" or higher scatter for it not to hit. At the risk of dragging us farther afield from the topic, I'll Mathhammer it, if you like.

2-9 (for inches of scatter) have a 30/36 or 5/6 chance to be rolled on 2D6 (that is, to hit), so with the chance to roll a "hit" on the scatter die, let's increase that to, say, ~8/9ths chance to hit. You need a 7+ on 2D6 to pen AV14: a 21/36ths chance to pen, then. From there, you need a 5+ on the Pen chart (before adding the +2 for AP1, since you need a result of 7 on a D6) to explode it: a 1/3 chance. (~8/9 to hit)*(21/36 to pen)*(1/3 to explode)=a ~17.28% chance to outright Explode a vehicle with that melta. And that's assuming no cover save or anything.