Well, the stats problem with MM3 is legit, it's a serious blemish on an otherwise great book. And the avalancher is goofy.
But, I think there are some differences here.
In particular, the dragonspawn, arguably the centerpiece of the book, given the bluespawn on the cover and the rampant cross-promotion(which is a good idea up to a point), just don't hold up well, especially with the silly spider-dragon hybrid and the iceskating whitespawn. Not necessarily a reason not to buy it, but when the centerpiece monsters don't hold up well, you've got a mark against the book. MM3 didn't have "iconic" monsters like the dragonspawn, or at least they weren't promoted in the same way.
The NPCs(and I took time to give the statblocks a perusal) are by and large uninspired, and are in fact something that a rules-fluent DM (or an NPC Designer user) could churn out rather quickly -- I've built more statistically interesting monsters myself. That's not to say there weren't a few cool ones -- the Yuan-ti Abomination with Marshal levels was a great idea(+7 to initiative and +7 to flanking damage anyone?) -- but on the whole, the concept didn't hit the places it could have. In contrast, MM3's "basic humanoids" were well-thought out and have held up fairly well -- flinds have been around for a while, and the lizardfolk types saw almost immediate use.
It's sad, really, because the lairs/ecology/treasure ideas were great, and deserve a better showcase. And the dragonspawn could have been nigh-iconic and gained serious traction if done right, but some of the monsters in that section would make a flail snail snicker.
Could the book still be worthwhile? Sure, it's possible -- I've already mentioned the Balhannoth as a really well-designed monster -- but I just don't see it happening. And I really don't like the idea of leaving the real stinkers out of the art gallery. Only time will tell, but I doubt it'll be as well-remembered as the MM3.