To me, it’s a scary experience and I could never be assured if the update was completed or not. Very often when I login to child sites I would find the plugin corrupted, deactivated or even removed from the site.
If there a way for me to view eg post update log/report so I’ll know what child sites I should check on?
Or maybe I shouldn’t use the update all function at all (I have about 70 sites at the moment)
The most comprehensive logs/reports are offered by a 3rd party extension Activity Log for MainWP.
However, it may not be able to catch all fail cases when a plugin/theme doesn’t get updated.
I’d say that if you have a large number of updates on each site, and 70 sites in total, perhaps you can try performing updates in two or three chunks, and observe if that helps.
If some of those 70 sites are on the same host, perhaps the server is being overloaded as well. To help alleviate that issue, try setting the values to default on the Dashboard > Settings > Advanced Settings page.