The cost of human labor to count all ballots by hand will be enormous. Probably worth it I suppose, but this really is something that should be primarily automated. But again, trust in software. Sigh, why can't we just have nice things?
What's important is being able to segment the population in enough voting places so that each voting place is maneaganle just by a small number of people. The Chilean system is scalable because you can always just add more voting places as the population grows.
Usually these voting places are civic centres, stadiums, schools.
It's a good system and generally for a presidential election we get the results in about 4 hours after voting ends.
In Taiwan, this is how it's done. Every ballot is counted by human. It's completely public: you can just walk in any polling station during the counting process and watch they count.
The US casts 10 times as many votes - so it seems reasonable for the US to hire 10 times as many poll workers? Hand-counting is O(n) i.e. constant per-capita, and it scales horizontally.
Local and state ballots in the US can feature tens of elected positions and propositions, I could imagine hand-counting them to be quite expensive.
In actually democratic countries the elections are done on holidays(Sunday) and the polling stations are in where you live.
It is your vote you silly. It is your democratic duty, right and responsibility to guard it if you don't trust the observers by becoming one. Everybody should be able to watch the process and the count!
Losing one day of revenue would not hurt. Especially on a holiday.