Both website and the Apps can be compromised.
This is actually how governments broke E2E-encrypted chat systems of criminals: by taking over the central server and from there updating everyone.
> Your account password is never stored alongside your 1Password data or transmitted over the network.
If you want a lot more detail, pages 10-11 of https://1passwordstatic.com/files/security/1password-white-p... are good places to start.
Sure, the legitimate client doesn't do that. But unless you do a full audit of 1Password's JavaScript every time you visit it to log in, an attacker who breaches their servers could switch out the legitimate client for an identical-looking malicious one that does do that.
(I have all my passwords in 1password, but if there is s legit weakness I'd definitely like to know it)
If you're really worried, use 2FA as much as possible and you can consider peppering your passwords.