Hey,
If the doc suggests that zookeeper is this kind of thing, then, I think that in the context of tooz this comment still reflects reality.
Unfortunately, I think that we can affirm that the main maintainers of tooz are now far away from the OpenStack world, and their deep knowledge disappeared with their departures.
I've no idea which driver is the most tested, but maybe you can run some comparisons by using the beagle command:
```
$ pip install beagle
$ beagle search --ignore-comments -f link --repo-pattern "openstack/*" 'zookeeper'
$ beagle search --ignore-comments -f link --repo-pattern "openstack/*" 'redis'
```
And then run some grep over them to catch the tooz use cases.
I ran the zookeeper one on my laptop and I found several use cases of tests in ironic among other components.
I'm far from a tooz/zookeeper expert, so do not take my response as an expert one.
Hopefully it will answer some of your questions.