"Onslaught" is not quite as terrible as many people say. Its problem is far more condemning: mediocrity.
Let's start with the team that is at the core of this story: the X-Men. The direction of Marvel's merry mutants was actually pretty solid at this point with Scott Lobdell finding a groove and Mark Waid taking over some writing duties. The immediate build-up to Onslaught - where we find out that Charles Xavier has gone mad and has betrayed the X-Men - is outstanding. There is palpable tension and a creepy air to the scene as Xavier calls his charges to his study, with only Jean Grey aware of what is happening. The creative minds brilliantly follows up on Jim Lee's left-open story about an X-Men traitor who - in Bishop's future - killed the entire team. The way the team survives is very smart.
But as soon as the Avengers and Fantastic Four get involved in the story, everything falls apart.
Marvel was desperate to revamp its main teams, to the point that they were about to hand the Avengers, FF, and Captain America and Iron Man titles to the very people who had abandoned them to form Image. And let's not beat around the bush about why these groups needed a refresh: their comics at the time were bad. There was some nice art by the likes of Mike Deodato (committing some horrible "90s art" offenses at the time, despite that) and Carlos Pacheco and Mark Waid writes a serviceable "Avengers." But with Tom DeFalco stumbling through Marvel's First Family issues here and "Avengers" coming off their worst story ever ("The Crossing,") these books needed to be taken out into the street and shot.
Tthe first "Fantastic Four" issue in this story is an example of how bad things had become. After Onslaught fails to kill his own team, the X-Men and FF regroup at Four Freedoms Plaza. It is here where Onslaught - who had as I just mentioned attempted to kill his own team - ambushes splinter groups all over the building. Now one would think that this supervillain who had - again - attempted to murder people would leap at the chance to do so when he has them dead to rights. But, for reasons that cannot be explained, Onslaught leaves everyone alive. It makes absolutely zero sense.
Such is the major problem with "Onslaught." For every scrap of momentum the X-Men series picks up, most of the tie-in issues in the rest of the Marvel Universe absolutely kills it. DeFalco and Howard Mackie's "Spider-Man" stumbles drunkenly into the fray with a completely forgettable fight against a few Sentinels. Then things get even worse in the short-lived (thank GOD for that) "Green Goblin" series. You have the perennially awful "X-Man" by Terry Kavanaugh, "X-Factor" during a particularly bad period (brainwashed Havoc, Random the traitor, Forge/Mystique romantic tension) and an unremarkable Punisher story all wedged in. Jeph Loeb's "Cable," Larry Hama's "Wolverine" and Peter David's "The Incredible Hulk" try to pick up the creative slack, but it's not enough to pull the crossover out of the mire.
By the time we finally get to a resolution, the Lobdell-plotted and Waid-scripted finale looks brilliant. There are some powerful moments as most of Earth's heroes sacrifice their lives to stop Onslaught. Things like Reed and Sue entering the breach holding hands, Quicksilver's realization that he's about to lose his wife, and Cap's final* "AVENGERS ASSEMBLE!" Compare that to the "Fantastic Four" issue that immediately precedes it when FF allies conveniently show up in the nick of time OVER AND OVER AND OVER AND OVER AGAIN. It's pathetic storytelling and makes you glad that Marvel gave all these characters a break.
See, this is why "Onslaught" makes me worry about the current state of Marvel. The company has just lost some of its best writers like Matt Fraction, Jonathan Hickman, Rick Remender and Kelly Sue DeConnick. While there is a lot of talent still scripting for the company, I worry that they're about to fall down the same breach that clearly plagued most titles during "Onslaught." Can the likes of Jason Aaron, Brian Michael Bendis, G. Willow Wilson and Mark Waid (AGAIN!) steer the ship when so much talent has gone elsewhere? I sure hope so.
Or else you eventually get crap on a near-company-killing level like "Onslaught."
Bonus note: I feel it's worth mentioning that there is a very enjoyable postscript issue included in the "Complete Onslaught Epic" books where the X-Men prepare to (and eventually do) eat breakfast. The sound Cyclops' optic blast makes as he shoots a flying pancake: "ZAKT."