Without question, John Byrne is the most versatile comic-book talent of the 20th century and can be forgiven for writing a handful of clunkers. Even Neil Young is forgiven one bad song per album. Let's refer to John Byrne as the Robert Plant of comics.
"Most versatile comic-book talent of the 20th century" covers a helluva lot of territory. More versatile than Will Eisner? Than Jack Kirby (taking into account his entire career, not just the tin-eared writing in his post-1970 work)? Than Walt Kelly (who did outstanding comic-book work before settling into a newspaper comics career)? Than Harvey Kurtzman? Than Carl Barks? Than Walter Simonson?
I cannot think of a single comic-book creator who can even hold a candle to the equi-partitioned talent in both writing and drawing at anytime during his/her long career; maybe Kirby but I suspect Stan the Man had a lot of say in the quality of his deliverables.
Alongside the stellar FF and Superman run, he is jointly responsible for the best story arc the X-Men will ever see, has been awarded two Eagle awards and has been inducted in the Comic Book Creator Hall of Fame. He is directly responsible for a quorum of Marvel readers defecting to DC, as followers of his work, leading to DCs claim to #1 in sales in 1986... a feat not matched since the late 50s.
No argument regarding Byrne's X-Men run (assuming you are referring to the Claremont-Byrne run, and not any later re-visitations); if those were the only comics he ever drew, Byrne would have to be acknowledged as a great comic-book artist. As for Fantastic Four, as a fan of an earlier version of the series, I had to stop reading FF when I realized that JB's handling of particular Lee-Kirby elements (e.g., Aunt Petunia) was ruining those elements for me when I went back to re-read the earlier classics. And I just can't see that his work on Superman was much of an achievement (in artistic terms, not commercial). As we now see on a regular basis, creators given free rein to wipe out what went before and start over can always goose fan interest for a few months.
I think he is responsible for bridging today's readers with most Silver Age readers; members of both camps praise his work so often and loudly.
His Next Men is often lauded with the same level of credibility and acceptance as another creator-owned product, that of Mignola's HellBoy.
Take away all these deeds, and we haven't even touched upon his artwork, which many, including myself, consider to be second to none. When I read John Byrne, it's money in the bank for me. His consistency is outright spectacular, and he has never missed a deadline. Not once.
On the few occasions I have recently looked at JB's new art, I have been surprised and dismayed at how little resemblance it bears to his outstanding work on X-Men. The current work seems scant in detail, and lacking in the vivid depictions of action and emotion that characterized his X-Men work.
With all this, I would not only qualify Byrne as an exemplary story-teller, but as the best writer in the business with a legacy that will last longer than just about anyone breaking onto the scene over the past decade.
Not having read a JB-written story in over 25 years, I probably ought to let this one pass, but.... "best writer in the business"? In a business that has had Steve Englehart, Alan Moore, Will Eisner, Neil Gaiman, Stan Lee, Carl Barks, Archie Goodwin, Harvey Kurtzman, John Broome, Jules Feiffer, and John Stanley? I'm afraid my mileage varies.
And then there is the matter of his personal pettiness, and his tendency to settle personal scores in the very public forum of the pages of his comic-book stories. Remember the issues of Uncanny X-Men, shortly after JB left the title, in which the X-Men encountered Dr. Doom? A character (Arcade, IIRC) struck a match on Doom's armor. Byrne, a few months later, wrote a story in which Doom was examining Doom robots, and noticed a scratch that appeared to have been caused by the striking of a match. The implication was that "Doom" in the X-Men story wasn't the actual Doom, but a robot, thereby unilaterally undoing a significant story point by his ex-collaborator (and by then, his enemy) Chris Claremont. I believe JB also portrayed an unflattering version of Jim Shooter in Legends (DC 1980s mini-series). I think there are other examples, but I can't bring the details to mind right now.
And he still delivers... The Atom (2006) saw him in good form.
I'm happy that you enjoy reading JB's work; just as I am almost always happy when I know that anyone is enjoying any comic anywhere. But it seems to me that the level of praise offered here is more extreme than the man's actual achievements warrant, and that it (perhaps unintentionally) denigrates work of other creators no less deserving of praise.