When update_on_save is active, and you make changes to a simple product related with a configurable one, there is a couple of problems:
The product is stored in the database for indexing (as it should), but when the cron job arrives and tries to take this products, it won't; and in consequence, it won't index any information related to it.
When this problem gets corrected, if this simple product price is changed, it will index it's new price but it won't do it with it's parent product, leading this to a potential unstable state of it's pricing. A possible solution would be to reindex the configurable product anytime a simple product belonging to a configurable one is indexed (by adding it to the table too).
When update_on_save is active, and you make changes to a simple product related with a configurable one, there is a couple of problems:
The product is stored in the database for indexing (as it should), but when the cron job arrives and tries to take this products, it won't; and in consequence, it won't index any information related to it.
When this problem gets corrected, if this simple product price is changed, it will index it's new price but it won't do it with it's parent product, leading this to a potential unstable state of it's pricing. A possible solution would be to reindex the configurable product anytime a simple product belonging to a configurable one is indexed (by adding it to the table too).