When he and I were kids UPA was an absolute sacred cow and you were ordered to love it or else by people who hated Bugs, Popeye and even Disney. Later in the 70's intelligent authors like Joe Adamson, Leonard Maltin and John Canemaker started covering the other side of the story and the playing field evened up, maybe to the point of overcompensation. Now it seems like Amid Amidi's excellent book CARTOON MODERN is almost overdue.
We don't always realize that the acceptance of WB, MGM and Fleischer cartoons as classics is a recent development. Up until the '70s, books on animation routinely dismissed all non-Disney Hollywood studio cartoons in about a paragraph.
UPA was formed in an atmosphere where the kind of work being done at Hollywood studios was seen as limiting, lacking in artistry, and too beholden to comedy formulas, broad movements, violent gags and funny animals. But now, the perception is different: the cartoons UPA was rebelling against have become canonized as the great masterpieces of animation. And so this has created a backlash against UPA cartoons, precisely because they were intended to be the exact opposite of the Bob Clampett/Tex Avery type of cartoon. I think that some animators who straddled both camps were a little taken aback by these reputation shifts.
But backlash is a poor basis for evaluating anything, and I don't think we can hold it against UPA for changing the look and approach of animated cartoons. By the time UPA got started, the basic template for a funny cartoon (funny animals, silent-movie-style physical comedy) had been pushed as far as it could go and somebody had to look for new things to do. I think many UPA backlashers incorrectly blame UPA for killing the Clampett/Avery-style cartoon, when UPA was really responding to early signs of the eventual death of that type of cartoon (including the fact that, with the studio system collapsing, a cartoon studio had to learn more from industrial/commercial animation in order to stay afloat).