Jython has historically lagged hard, often falling behind for very extended periods. For a time their releases basically just stopped, which led to them missing support for pretty much anything between 2.7 and 3.6 (iirc). I know the project basically rebooted at some point, but I've since lost interest.

Not to mention the biggest drawback imho. Those alternative implementations don't support C extensions.

Jython was dead for a long time. It might be back a little now, but there is still no Python 3 support.

GraalPy is much more active and more compatible.

Not me, someone else in the company did, I don’t remember why it was dismissed.