Oral contract closure: It isn't easy to prove because you need a witness
Simple electronic signature (SES), e.g., a scanned signature image: you have the same issue verifying that the contract's signature is valid. If it comes to a court case, the signature must be analyzed. Due to the weakness of the procedure, witnesses of other persons or other hints will play a significant role in proving a power of attorney.
Advanced electronic signature (AES): AES has no legal basis. Hence, the contract parties have to go to court again. The court will ask a specialist to investigate the AES of Swisscom. Due to the audits Swisscom performed, the specialist can benefit from it. Nevertheless, the AES is weaker than the QES, e.g., concerning the way to identify/register persons, the time of archival (only seven years), and the 1-factor authentication for the signature in contrast to 2-factor authentication (thus, a stolen mobile smartphone could be used for a signature).
Qualified electronic signature (QES): Other than SES and AES, you can verify a valid QES with a validator, e.g., a Validator by the Swiss Federal Administration or Signature verification of the Austrian Telecommunication and Postal Division (RTR). Contract parties do not need to go to court in case of doubt. Only if somebody generally doubts the audited trust service will this be hard proof. QES signature evidence and logs will be stored as long as foreseen for all commercial and tax documents in the business record: more than ten years in Switzerland and 35 years in the EU/EEA.