For online voting, someone can log into the site/app and then hand their device to someone who bribed or threatened them. For an in person vote, you can mark the ballot one way and then tell someone you voted another way.
Also, anything done with computers is easier to automate, so you could take over a million devices with a virus, or just hack into the server where all the votes were stored.
if you were mailed a copy of your vote later and asked to verify it, you could just say it was invalid or correct it via a mail in response form, if you had been threatened
This is just moving the problem around. The briber/threatener can just demand that you request one of these correction forms and fill it in for you.
Having said that, these risks also apply to postal voting, so the jurisdictions where that is common have either been lucky or haven't noticed their elections being subverted.
Elections are something you have to get right every time, because if an attacker manages to find a flaw in an election process and gain power, they can prevent anyone else from being able to patch that flaw (and can also corrupt the system further to their advantage).
well ideally i think you would design your constitution/institutions such that it takes more than a simple majority to overturn the system. no system is perfect, everything has tolerances.
So everyone can prove to the person who bribed or threatened them that they voted how they were told to?