I doubt Google Classroom itself is the problem at all, that's pretty rock-solid in my experience.

But with third-party auth, the third-party add-ons and integrations, that's where it turns into complete crap. Those are the things that often tend to be terribly engineered.

Classroom pretty much works like a dream if you stick to materials in Docs/Sheets/Slides and do the grading within Classroom as well. And if each student has their own device so they don't need to log in again every day, even better.

But schools purchase all of these integrations and then expect the teachers to use them and then the quality of the user experience goes down the toilet. But the fault is pretty much entirely with the integrations, in my experience. Because some are quite reliable, it can definitely be done. But a lot of these companies just don't care.