As far as what can be considered a straight up scam, those are actually pretty rare and easy to spot.
These are the kind where everything is fake, the team, the company, the website.
Nothing of it exists and the after the ico they disapear and run away with your money.
If you do even a simple amount of research, you can quickly see if it is a completely fake company or not.
What most people nowadays call scams, are really just failed business ventures and startups.
What is more common is that a startup raises funds through an ICO, and the business goes nowhere, and the value of the coin or token drops below ICO price and stays there.
The most common reason why ICO startups fail is because of an incompetent tech team.
A lot of these companies gain no benefit having blockchain and they can't even implement it well.
No progress is made and the whole thing just dies.
You have to realize, that tokens and coins are not stocks in the company, if they have no real use then the value will fall.
I quite agree with you to some extent. However, there are projects which are intentionally dumped in the market by the devs. How do we explain the case of Swisscoin (SIC) whose total supply was increased few days after it came to coinexchange as the devs pushed more coins into the market. That act was a pure scam and many holders quickly sold off causing price to dip greatly.