True, in their early years Schoenberg and Stravinsky inspired the bloodiest riots ever seen in the concert hall. But I think as the 20th century went on, critics' hearts weren't really in the grand abuse anymore. I also have a hunch that after Slonimsky published The Lexicon of Musical Invective, critics acquired a collective anxiety about appearing in the next edition. You don't want, like Joseph II re: Mozart, to be in print as a philistine for the ages.
In the prelude to his Lexicon, Slonimsky arranges his kitchen full of pans into themes: gastrointestinal, animal, anti-Semitic, and so on. And he proposes a general theory of acceptance of the unfamiliar: "It takes approximately twenty years to make an artistic curiosity out of a modernistic monstrosity; and another twenty to elevate it to a masterpiece."
Therein lies a fundamental shortcoming of the Lexicon. Reading it, one absorbs an impression that actually isn't the case: that great composers get only bad reviews and are appreciated only after they're dead. Stepping back from the melee, one discovers that while some splendid composers do take decades to sink in (and Schoenberg never entirely has), more often the true revolutionists of the past were hailed for their imagination, and their most radical pieces were quick to find an audience. Everybody knows about the pandemonium The Rite of Spring provoked at its Paris premiere. Few notice that the screaming had as much to do with Nijinsky's choreography as the music, and that after a concert performance of the Rite a year later, Stravinsky was carried through the streets of Paris on the shoulders of a cheering crowd. An earlier epochal work, Beethoven's Eroica symphony of 1803, was greeted by a chorus of incomprehension. But only two years after its premiere, the leading German musical journal declared Eroica "one of the most original, most sublime, and most profound products the entire genre of music has exhibited." Meanwhile, Slonimsky's Lexicon encouraged composers in their delusion that scabrous reviews are a badge of honor, that if you aren't denounced you aren't any good. When all is said and done, I'd wager that through history the majority of lousy reviews have been bestowed on lousy pieces, but nobody collects the notices of forgotten composers.
Still, Emperor Joseph was a dope, right? Not at all. Joseph was a capable amateur pianist and intimately knowledgeable about music. What he said to Mozart was what everybody said: too effusive, too many notes. The thing is, they were not entirely wrong. Mozart's operas are full of stunning throwaways. There's a heart-stopping orchestral eruption in the middle of The Marriage of Figaro that is evoked by nothing but a woman's name, Marcellina; in the story, there's no reason for anything nearly that glorious. It drove other composers of the time crazy that Mozart could toss off bits that were more beautiful than anything they ever wrote. (It was the arrival of Beethoven that made Mozart's notes seem frugal by comparison.)
On the whole, Mozart's critics viewed him about the same way we do, as an incomparable genius, though not an infallible one. One critic lambasted Don Giovanni for a story that "insults morality, and treads wickedly upon virtue and feeling." But let's face it, the opera is on the amoral side. (It's just that these days, unlike the 18th century, we like amoral.) As for the music, the critic went on, "If ever a nation could take pride in one of her sons, so Germany must be proud of Mozart. … Never before was the greatness of the human spirit so tangible, and never has the art of composition been raised to such heights!" Even some of Mozart's bad reviews called him the greatest composer who ever lived.
Really, this is a lament for a lost era. The great lousy reviews arose because critics and audiences truly cared about music and its future. Critics were sometimes reactionary, boneheaded, and cockamamie, but music mattered to them. If we no longer enjoy the uproars and the withering screeds of yesteryear, it's mainly because people no longer care passionately enough about what they hear in the concert hall to want to murder somebody over it.