We say SQLAlchemy support is “beta” to mean that not all features will work. However, we are already committed to continue to support those SQLAlchemy features that are already supported. If you find the current support to be adequate for your use case, you can rely on it safely.
Perhaps the word “beta” was inappropriate in the tutorial; a better word would be “incomplete” or “partial”.
As to the other point: Spanner’s originally did not support INSERT/UPDATE/DELETE to modify records, which in turn severely limited its use in 3rd party tools included by ORMs. Moreover, CockroachDB supports the PostgreSQL language dialect, which is widely supported by ORMs and thus facilitates migrations. Spanner uses a separate dialect which limits its compatibility. There is no more to take away from this comparison.
We will write more documentation on the topic of 3rd compatibility and the stability thereof before our next major release.