on-disk format/write amplification: > For tables with a large number of secondary indexes, these superfluous steps can cause enormous inefficiencies. For instance, if we have a table with a dozen indexes defined on it, an update to a field that is only covered by a single index must be propagated into all 12 indexes to reflect the ctid for the new row.
How wide is their data? Depending on the answer to this, it could be that they've over-indexed, have a poor indexing strategy, or are reacting to the poor queries generated by an ORM (not sure if they use one, or if they hand-code their own SQL).
data corruption: everyone has bugs. https://bugs.mysql.com/search.php?search_for=&status=Active&... add in mysql's tendency to loosely-adhere to the SQL standard and there are many ways that you can actually corrupt your own data.
i'm not here to debate whether postgres is better than mysql. i'm just saying that it seems like a lot of research went into justifying a switch. who knows, maybe that research could have been spent optimizing their current environment.