For something to fail means it started at some point right? Now in my own opinion, a failed project is a project whose team started the development process, worked for some time and then it came crashing maybe as a result of lack of further ideas, mismanagement of funds or not sticking to their roadmap thus leading to lose of interest within the community hence failing. Also, a project depreciating within time is majorly caused by the team, how? As a project team, they have to put everything in place to ensure the continuous developement of the platform, they also need to follow their roadmap, adjust when need be, add more features and so on, through this way, they will constantly keep their users hoping for more hence more engagement from the users and that's what drives a platform to be valuable; take a look at Binance platform, Ethereum etc, then you will understand their teams are different from a whole lot of other projects. A scam project is one which didn't developed anything but ran away with the funds generated.
Lastly, a potential project have the neccessary features, but first they have a very good use case and by mare getting involved with the project, every idea and what the team is working on is laid bare to understand. A project with potential also goes for good exchanges so as to attract more users hence keeping the development going.