Having monitored physical polling stations where people vote in secret removes this opportunity, because I can only see that you voted, but not what you voted for.
In practice I don't think there are even fringe accusations of it being a problem.
> Having monitored physical polling stations where people vote in secret removes this opportunity
It reduces, but does not remove, the opportunity. https://en.wikipedia.org/wiki/New_Black_Panther_Party_voter_...
Currently.
> You wouldn't be able to realistically intimidate 10,000 people and watch each one drop their ballot into a box.
Your fanatical followers and/or henchman can do that. You can also use photos/videos to watch "your" voters more scalably. Another option is to pay them rather than intimidating.
The risks should be weighed against the benefits of greater turnout, but countries and even US states have found ways to improve turnout while keeping the safeguards of the secret ballot.
Before 1932, the vote in Brazil was not secret. It resulted in widespread voter coercion and bribery, a system we used to call "halter voting". The "colonel" (powerful landowner or authority figure) would use the threat of violence to force those in his electoral "corral" to vote as he pleased.