Eh, MacKinnon is 20 years old, bit early to call him a letdown. He came in and had a fantastic rookie season being extremely young (was 15 days away from being drafted the following year). After that he had a sophomore slump as most guys do. Didn't help Stastny walked either as MacK, him and Landeskog had terrific chemistry. Overall Avs have just been a bad team so I don't know how you can draw any real conclusions on MacK yet. 21 goals and 52 points this year in 72 games isn't exactly bad either, even though I expect him to outproduce that in the coming years.
Huberdeau is 22 and has just put up 2 straight good seasons and he wasn't even that hyped, it was just a weak draft year. He, too, have played on a bad team and I think it was expected that he would have some difficulty translating his game to the NHL game, but I'd say he's right on track. In fact I'd say he has positively surprised me because I really didn't have that high expectations on him but he's having a pretty good season this year.
Ehlers is technically still a junior but have had a respectable rookie campaign and should only get better. Drouin obviously isn't where most people thought he'd be at this point but it's certainly not due to lack of talent. Rather it's due to a bad attitude and, well, the fact that Tampa is pretty stacked up front. It's been really hard for him to get quality minutes. Still, he's 21 years old and he's got plenty of time to prove himself to be a very productive forward in the NHL.
So really, you can't say the Q is weak or that you don't trust players drafted from the Q based off of any of these players.