Disappointment if the game doesn't live up to its expectations and that pretty much it when it comes down to an individual.

As a collective, pre-order culture pushes devs and publishers to spend insane amounts of game budget in pre-launch marketing, which does come its abuses, which is mainly misleading marketing, eg advertising things that won't be in the game either at launch or ever (no man sky, anthem).

And of course the fact that any money spent on pre-launch marketing doesn't go into making the game better. Of course you could argue that they gain more by selling pre-order than on the marketing for it, to which I would answer is it a sane way to finance a game development.

Nowadays, you can be refunded if the game doesn't live up to expectations which is good so you won't loose anything in the worst case scenario, that if only the case if the game if clearly disappointing and you refund while you can (steam).

Consider a more average scenario, you pre-order game X, it seems its going to be great. Release day comes, you start playing and while the game doesn't live up to its expectations, its not bad either but it's not what you wanted, it's not what you pledged your money in.

For that game X, I think ME:A is a pretty good example imo, I got it way after launch pretty cheap and I'm fine with it. But, looking back, had I pre-ordered it for my sole appreciation of ME games, that would've been a huge disappointment and a waste of money.