The problem I'm describing isn't really the ORM libraries' fault as its outside the scope (hah). Your library is great. Most of the trouble I see is where the ORM meets the web framework. I think that gets tricky for lower experience developers because it's fundamentally tricky in Python. What the scheduler (e.g. gunicorn) does and how it forks and how sessions are handled at the wsgi layer are where you have to be very careful. Django has brought that in scope and you haven't -- which is why thats a better wholistic webdev experience and SQLAlchemy is the more powerful ORM.
With that said I stand by my comment having read the full changelog (commendably thorough btw). Unless you can afford to figure those things out as a team, it would behoove most web devs to reach for Django first.