Lets look at the wraithknight profile. T8, W6 3+ is pretty resilient. It is a jump monsterous creature, so it can move 12" a turn. Shooting it has some options, but it is well known for its two S10 shots at range 36". In combat it is S10, 4 attacks at Initiative 5. Overall pretty impressive for just about a landraider in cost.
Now lets see the Tyranid Scythed Hierodule. T8, W6 3+. It can move up to 12" a turn. For shooting it has a S6 Ap3 Apoc Flame template. In combat it is S10, 7 attacks at Initiative 3. It does get to reroll to hit because of Scything talons.
As you can see, there is hardly any difference in their profile. The shooting comes across different, but neither really outshine the other since they do different rolls. So why does the Hierodule cost 350 more points than the wraithknight?
The hierodule gets immunity to sniper and poison weapons (they only work on a 6+) but fleshbane still works. He also is immune to instant death. He can charge a unit that he didn't target with shooting, which is nice, but he loses the ability to fire overwatch. The real winner though is he gains FNP for free and the Stomp attack. Stomp attack is an extra attack, that can help thin large units. It is not as useful for a combat monster like this, but still can be very helpful.
But does all of that equate to 350 points worth of special rules? I think in most cases 2 wraithknights would be the better option. This just kind of shows how sometimes the point costs really don't match up right.
Funny last note, A wraithknight could get removed from play from the Stomp table, as it isn't a gargantuan creature and is not immune to that. Even though it has the same stat line as the creature performing it, and is a physically larger model.