Assisted logins are any "Login with ____" options. Google, Facebook, Apple, et al offer this. Its a bad deal because they (to a degree) take control away from you and contribute to further unnecessary tracking. I have anecdotally also found it to be annoying in terms of disabling if I changed my mind and wanted to use my own username/address and generated password to more directly control my account associated with it.

Additionally, if your account with the assistee was ever terminated or restricted, you may have a bad time trying to (as I remarked about) get back in direct control of your account. I don't like having all these things daisy-chained. Its like to digital version of being under conservatorship or babysat. If you're not prone to phishing I would advise against it, although I would prefer resolving that cardinal sin in shorter order where possible and practicable.