My understanding is that the current approach for e-voting in Switzerland works with voters being sent a PIN in the mail [1]. Then the voter uses the PIN to log into the system and vote. Unfortunately that means that insecurely discarded letters from non-voters could be used to cast votes on behalf of these voters.
Some jurisdictions try to use a second factor to prevent these attacks. In Ontario, for example, many municipalities use a combination of the voter's PIN in the mail + the voter's date of birth. But a date of birth isn't really secret. Lots of people know your date of birth (especially insiders at organizations that collect this data), and it may appear in data breaches or even publicly on social media. If you're curious about this, I recently co-authored a paper which is all about security problems relating to online voting credentials in Ontario -- It's relevant to the Swiss case as well: https://link.springer.com/chapter/10.1007/978-3-032-05036-6_...
Long story short, using a digital ID to authenticate to the system (like Estonia does) goes a long way to mitigate this authentication problem. However, there are still plenty of other potential risks with online voting that are unrelated to authentication (how do you prevent ballot stuffing, clientside-vote-altering malware, falsified counts, etc). And there may be privacy risks with digital ID depending on the practical implementation.
[1] https://digital-solutions.post.ch/en/e-governmenthttps/digit...