To be fair, I think people are overly hard on Millennium, but the criticisms seem to come from two separate camps: those who can't get past the production style and those who unfairly require it to top their self-titled US debut and their first two international albums. I don't think either argument is fair.
First, regarding the production, we've heard this kind of thing before. People still criticize Springsteen's production on Born in the USA, saying it was too synth heavy. But if you're willing to embrace the era, it's truly great and likewise, Millennium is a real gem. At the very least one should try to put aside the bias against the production and listen to the songwriting and the artistry in the vocals in order to enjoy it despite the perceived shortcomings in production.
But I actually take the complete opposite view: that pop albums in circa-1999 were in their absolute prime. It was before autotune, but after protools and so you get the best of both worlds: perfect rhythm and a tight musical sound paired with a casual, naturalistic vocal. Sure, it doesn't have the flourishes it might have if someone like T-Pain or Timbaland had been involved, but it has its own charm for sure.
Second, regarding the comparisons to their previous work, I'm not sure how that's fair at all. Artists can't be required to replicate their previous work. They need to be allowed to grow. I have no doubt that a group as talented as the Backstreet Boys could rest on their laurels and put out an album full of tracks that match the fire of "Everybody (Backstreet's Back)". They should focus on putting out the most honest music that reflects where they are in their development as artists. To the Boys's credit, that's exactly what they did.
Millennium is full of songs that express a mature view of relationships. In fact, their maturation is so striking that I'd actually argue, if anything, it actually eclipses what came before it rather than falls short of their previous hits. Sure, "Quite Playing Games With My Heart" and "As Long As You Love Me" were great songs, but they were fairly straightforward in their view of love. I could hear any number of pop singers doing those tracks justice. But no other artist on the planet could possibly give the depth of meaning the Boys gave to "I Want It That Way" and "Show Me the Meaning of Being Lonely." I could parse the lyrics all day for the subtle shades of experience, but the fact that they were drawn to this material shows how they had grown and that's what makes Millennium so special.
So give it another chance. If you have the time, listen to the self-titled US debut and Millennium one after the other. If you still think that Millennium is a disappointment, fair enough, but I think if you have an open mind you'll have a hard time pressing stop on your Discman.