The biggest mistakes developers make that leads to bad ending for many projects are
1. Huge Discounts
Many new projects are still making this mistake over and over again, huge discounts like 50% is insane and it always bother me how the hell the early buyers won't end up dumping since they have already see big profits.
2. Lack Of Solid Products and Ideas
Lack of working products and real use cases is why many investors are not taking new projects very seriously, no one would want to hold such tokens or coins for long, developers need to start working on how their project's token or coin will be very demanding throughout the globe
3. Bad Exchanges
Imagine new projects that make over millions of dollars in ico or ieo and end up listing on bad exchanges with no enough volume, what do they expect to happen?
There is no better truth than all you just pointed out. Most projects thinks it's by giving huge discounts or bonuses will they be able to attract investors but it always has a negative effect on the price of the coin thereafter. Also, most projects doesn't last long or even for a reasonable amount of time owing to lack of ideas to keep driving the platform; in the same way, some projects are just a copy of already existing projects, therefore failing as well. Bad exchanges also contributes which is sometimes as a result of the team trying to enrich themselves although there can be other reasons too.