Even in FAANG, most data is going to be extremely messy.
* There's usually very little incentive for good documentation, so you have to guess what the table contains based on column names and exploring patterns in the data.
* Enforcing a single pattern/process for data across the entire org is a pipe dream.
* Logging tables are written to by hundreds of engineers across dozens of teams, with no standardized naming scheme.
* The product contains multiple edge cases/special overrides for very specific circumstances, so there's very few simple queries that don't come with 100 footnotes attached.
FAANG is not immune to large-organization problems, and data quality is no exception.