These “earn cryptos” projects usually give you their own coins or tokens. Most of the time, they don’t really get any value so you are just stuck with worthless coin or token.
For most of them, it's true. That they are unsuccessful and that's why they've just rewarded you with a worthless coin or tokens. But for some instance, there are these projects that becomes successful and the distribution that they've given as a reward has a value.
You can try it but I don’t think it will produce much profit.
There is no harm in trying but it's best not to put your much attention on it and if they're asking for some investment, you shouldn't be too optimistic with them because for the history of these projects, not all are profitable.