For a gold standard election you should:
1) Be able to identify the correct people and allow them to vote.
2) Give them certainty that their vote is recorded correctly.
3) Give them privacy while voting so that they cannot be coerced or bribed (postal voting fails this one too).
4) Allow volunteers to audit the process. e.g. Observe polling stations, sealed ballot boxes, numbered, signed, counts open to observers, tallies checkable, votes recountable etc.
5) Allow wide access to the vote, short queues, local polling stations, physical access etc. Postal voting helps this but at a cost to points 3 and 4. This is manageable when postal voting is a small proportion of the total but gets riskier if it rises.
Online voting fails to allow for most of these requirements in ways that are not just hard they are impossible to fix. You can trade some for others like the ability to confirm your vote was correctly recorded but that gives a receipt and the possibility for coercion or bribery.
Now the US voting system also fails many of these at least in some locations but giving the power to run the elections to controllers of the computers is too dangerous. There are just too many opportunities to tamper with votes that way with almost no risk of detection.
I had not looked into the Estonian experience until now but this is one perspective on it:
https://www.verifiedvoting.org/report-on-the-estonian-intern...
The verifiedvoting.org website also has lots of other information about internet voting and voting machines generally. I did not look at it before compiling my list above.