The ability for poor implementations to exist is itself a strike against the general concept.
Have to load money, have to retrieve money, and have zero insight into how much I’ve already spent without even more apps or interfaces.
Handling cash is a hassle in many ways:
1. It's slow. Obviously it is not slow in the best case when the customer throws in the exact change, but it is _really_ slow in the probably more typical case when the drunkard is at the counter and starts to dig into the pockets and find out if he has enough coins or when the seventh customer in the row wants to pay his ten cent lollipop with a hundred dollar bill and you run out of change.
2. It ties capital to keep sufficient amount of change in all of the booths.
3. It is difficult/expensive to manage securely and safely. You need to figure out how to stop personnel and other people stealing the money. Also your personnel likely feels more safe when they know that nobody can think that hey can come and try to steal money from you because you simply have none.
Or if I'm feeling extra tech-y, I can do that with my watch or phone.
2. A negligible amount for a vendor on a festival.
3. Not a problem on a festival. Booths tend to be manned by two persons because of other safety factors anyway, so staff stealing isn't an issue, and any organised criminality on a festival is usually limited to (relatively) low risk, high reward crime: pick-pocketing (smartphones in particular).
And especially the ‘takes longer’ argument just shifts the time people are waiting to the charging station, or to their own homes, if they’re charging beforehand.
I think it’s fairly easy to make any change handling easy by making everything cost multiples of $1
Even if some transactions have extra friction such as ID checks for age-restricted purchases, the less intrusive solution is color coded "dumb paper" wristbands.
have zero insight into how much I’ve already spent
If every single actual implementation of an idea is bad then maybe the idea wasn’t such a great one either.
Contactless payments with a regular credit card are as far as this idea needed to go, everything else is either a gimmick or a rip-off attempt.
If that’s because the festival organizers don’t let you retrieve your money easily or without a cost, that’s an issue with the festival management and not with the idea of digital money.
I’ve been to a festival which allowed us to withdraw money freely (I think they had some restrictions such as only 1 free withdrawal after the festival was over, and/or it had to be done within a week or so) but that allowed us to simply load in excess money so we never had to recharge. That worked really well for us.