Well the Onager is undoubtably the weakest of the three. if you ever wante an example of how not to use iJJs, this is it. I'm a big fan of HAGs, but they are essentially a mid-to-short range LRM with extra efficiency and no exploding ammo. The Pariah used the HAG right. Give a mobile platform a hole puncher, and a BFG HAG to chew away armor and seek crits. The Onager did none of that. The Onager isn't useless, and at least looks pretty cool, but it's certainly not something I'd want to bring into play for anything other than as an oversized crit-seeker.
The Omen i haven't gotten a chance to use yet, but it seems to have the most effective weapons array of the group for general work. Definitely a knife fighter by preference, it'd be fun to see what it can do in hilly terrain or a town.
The Night Wolf, on the other hand, is a fun eclectic mix and match of a mech. The Albatross comparison is apt, but I'd argue it definitely has more of a role in your average army than that fun-but-maligned mech does. The Plasma Cannon is one of those weapons people tend to forget how effective it can be. The same people who have been carrying around volatile inferno ammo for years will immediately shun the PC as the red-headed step brother of the Plasma Rifle. What they neglect to take into account is how much a potential 12 extra heat can throw off even the most efficient, heat neutral designs. It's not the perfect qeapon, but between it's absolutely devastating effect on infantry and battle armor, to its ability to completely throw off the heat curve of heat tracking units, it definitely has its uses. The rest of the weapons loadout is also geared for close range damage. HLLs has some range but with the natural negation of the Targeting Comp you should really use it up close, and the HE ammo for ATMs gives you absolutely no reason to try and stay at range. And while it may not be quick enough to necessarily dictate range, a 4/6/4 assault mech is definitely fast enough to at least get into position without too terrible a headache assuming you are using Floating Battlefield. All in all, as was discussed in the MotW thread, the Night Wolf is a surprisingly effective design all things considered. Not my first choice for a clan assault, but it definitely can contribute to any force.