Meh, modules aren't the game. You can have a good game with bad modules and vice versa.
(And, BTW, thanks for saying 'modules' instead of 'adventures' or 'APs,' makes me feel less ancient.)
A module does prep for you, but, IMHO, prep just isn't as valuable in 5e as it may have been in prior eds, and it's certainly not easy, either. So, yeah, buy a module so you don't have to do prep that's hard, I get. But prep isn't that useful, either, you're still going to have to react, improvise, & make rulings, regardless, and every time you do there's a chance to inject a little of the balance or challenge or whatever it is you were hoping the 6-8 encounter guideline might deliver.
Levels still have meaning even in a 5MWD, it's just that meaning is more hps/damage rather than bonuses/DCs - that's a matter of BA.
And it's not the core balancing mechanism, I'm not sure it's even a mechanism. It's a guideline that's supposed to help with balance, but balance is still primarily the DM's responsibility - like everything else, that's DM Empowerment, taking responsibility for the success of your campaign. You can't blame D&D for being imbalanced, it's virtually always been imbalanced, if it weren't, it wouldn't be D&D. Just be thankful that there's even that theoretical guideline, at all, it's one of the playtest promises that was actually delivered on, afterall.
LOL I guess that takes care of any problems I might have with the game. Just have the DM take care of anything missing or imbalanced, its his job anyway, not the game designers - got it.