The issue with the hype players, is that the ones that actually reach their potential far outsell even their initial hype prices. For example, everyone though people were crazy paying $200-300 for a McDavid YG on release or $80 for a MacKinnon YG...now those prices seem like bargains.
For hockey, it's always the same thing. The hot YGs sell for $70-200 or so. Then they drop in price. Then will either fade into almost nothingness or shoot up in price dramatically. Picking the best rookies to invest in is the trick, and it's a tough one to accomplish. There's a reason why the market sets the prices as they do. In 2013/14, the hottest YGs were: MacKinnon, Galchenyuk, Yakupov, and Huberdeau. Obviously looking back on things now, we can see the vastly different paths these 4 players took. Yakupov and Galchenyuk are worthless. MacKinnon stayed somewhat strong before skyrocketing. Huberdeau tanked, but has now started skyrocketing again.
I don't think it's as simple as rookie cards are a bad investment. In fact, they are the best investment. It's a matter of separating hype and actually picking the good players though. Also market timing. Not all rookies are best to buy right out of the gate.