Well, here're the advantages:
You get Fear, Hammer of Wrath, Invincible Behemoth, Move Through Cover, Relentless, Smash, Strikedown.
That's pretty impressive... until you remember that Monstrous Creatures have Fear, Hammer of Wrath, Move Through Cover, Relentless, and Smash, so the benefits to both are practically identical.
The Super Heavy Walker has a Stomp attack, sure, and most of the time that'll be Str 6, AP 4 hits. That's pretty good...
...until again you remember Invincible Behemoth isn't an advantage in comparison to Monstrous Creatures, but a weakness. It means that whenever a weapon rolls a Penetrating Hit against you, and rolls a 6 (5+ with an AP 2 weapon or 4+ with an AP 1 weapon), you sustain an additional d3 Hull Points. There're no circumstances under which Monstrous Creatures sustain an additional d3 Wounds from a weapon attack.
So are you a little better in hand-to-hand? Yes, thanks to Stomp attack, but you're also far weaker against ranged weapon fire, as each Penetrating Hit always has that chance of rolling a 6+ for damage and inflicting an extra d3 Hull Points on you. And at 6 Hull Points, it's entirely possible to be killed in just two shots, and it's highly likely if you're getting hit by AP1 weapons. Meanwhile, the Wraithknight and Riptide can be hit by those same weapons and will only ever sustain at most 1 Wound from such weapons. They won't ever take 2-4 Wounds from a single blast.
On balance, the Super Heavy Walker is weaker than the Monstrous Creature, particularly when you take into account the fact that the Riptide has a 2+ Save (Krak Missiles can bounce right off it while they can kill the Knight) and the Wraithknight has a 3+ Save, and the Riptide has a 5+ Invulnerable Save which can be boosted to a 3+ Invulnerable Save, and the Wraithknight can take a 5+ Invulnerable Save, while the Knight has an extremely limited 4+ Invulnerable Save which is only good against some shooting and is totally worthless in melee combat.
