polylang is stable now?
I am aware of only a few people who have encountered issues during the upgrade to 1.2 or older, always due to conflicts with other plugins.
That's a major upgrade in both program structure and database. So it's more risky than usual and that's why I recommended (everywhere I could) to make a database backup before upgrade (to ease revert if needed). And I still recommend it.
However nobody lost his data because Polylang never removed them (the upgrade process is creating data for 1.2 and newer but does not remove old data from 1.1.6 and older: I will remove them later). That permitted some users to revert temporarily to 1.1.6 using that plugin. But making a DB backup is safer.
Since 1.2.3 has been released, no more people has complained about broken upgrade.
I will never claim that Polylang is bug free. And there are always possibilities of a conflict with a theme or another plugin.