Even at the best of times, at his most commercial, Lou Reed is nearly unlistenable. His best efforts didn't get past the Electric Banana, really, if you're into pop or rock. That's not to say there's not lots of good VU music after the Electric Banana album, and that we should all applaud artists like Lou Reed and Metallica who want to try something different.
But why, when you're trying to be avant garde and stretch boundaries and the like, would you release it to your fans and have rock critics review it? Its not rock or pop. Rock and pop fans are addicted to (a) experiences of intense flow states that last less than five minutes, especially if those flow states are accompanied by dancing, but they may go as high as nine minutes if aided by recreational chemicals, and (b) the purchase of little plastic discs that represent to them the physical substance of their flow states. The rock/pop music industry only cares about (a) to the extent that it increases the rate of change on (b).
People who are fans of other kinds of "popular" music (not rock or pop, but popular, i.e. not classical) may more or less desire (a), because you can certainly induce a powerful and analgesic state of flow listening to So What (warning: the video begins with an ad for the aforementioned Reed/Hammett colloboration), but since each version is completely live and generally north of 8 minutes and they're all very different its not quite the same as listening to a very catchy set of remixes of Elvis or the very popular Verve Remixed series. That's pop. (And this is the future of pop.) And blues isn't pop either; Rory Block points out that Robert Johnson arrangements haven't been improved in since the 1920s, but while they may have danced to that stuff in 1920 all those seventh chords make blues far from analgesic. You get a flow state from blues, but its not the cocaine rush of pop, its the "why did I drink so much scotch" flow of trying to get home at 2 AM just drunk enough to wish you were with someone else.
Pop claims its going to change your life, take you on long trips to gorgeous exotic places where you'll learn wisdom and the secrets of controlling the universe, but really you're still at a 7-11 with your friends. Pop made cultural literacy a commodity, so that kids at the 7-11 could figure they know what's going on in the world, even though their art signifies absolutely nothing about the world. Pop music is like sports that way: people care deeply whether this team or that team wins, but it doesn't mean anything. Or it does mean something, but the meaning goes no further than the ability of various commercial media to place ads on time and space where they can be certain someone will be watching. Pop music bears the same relationship to the musical arts as the Sports section bears to the business pages.
So I would hope, in the grand scheme of things, that Lou and Metallica don't consider the critical reception disappointing. It disappointed a bunch of people who expected five minutes of flow and got challenged - that's not a surprise. (Not that I'm going to listen to it, because I'm not a fan of either.) If Lou and Metallica have somehow built enough cultural and financial capital with their record companies to put out an album that will sell nothing but may be viewed fifty years from now as the Kind of Blue of modern pop, then good for them. Someone has to do it.