For me, the convenience of the passwords app outweighs any of the potential security risks. As far as the user password, I would make a Guest account for any access at a tech shop. That is what the Apple Store asks you to do for Genius Bar repairs. Even if the Admin password for your account is changed from the Guest Admin account, you'll have to reauthorize the passwords app with your iCloud password, which you should never give out to anyone.
Furthermore, if something was compromised, the most important accounts all have 2-factor authentication with my authenticator app. SMS authentication is less secure, especially in the scenario where a computer shop has your Mac that can receive all your messages.
Storing passwords anywhere always comes with a security risk. Big companies (Especially password managers putting a target on their back) can always be breached, and I find that scenario more likely to occur than a third-party computer shop stealing your passwords when you bring in your computer for a repair.