It's not a matter of complexity, it's a matter of power, or rather, information asymmetry.
Buying stocks isn't really investing, it's more akin to gambling like Poker, with all other investors at the table.
Many of those investors have massive computers, R&D firms, brilliant minds, 'inside information', and relationships with those companies and CEO's.
So what 'millenial' is going to trade better than those?
Zero. Or at least in the long run.
The only way to 'beat the man' is to have more knowledge than Wall St. and that is extremely rare.
So what 'Robin Hood' does is sign up fish to feed to sharks.
It might make sense to put a chunk of savings in a broad array of stocks (some in bonds, some in gold, some in cash etc.) - but 'trading' against Wall Street is about as smart as playing Poker against the best in the world thinking you're going to win.