The challenge of syncing from stubborn SaaS tools to your data warehouse / database I suspect is different than syncing data from your data warehouse / database back to SaaS tools. Specifically, reverse ETL has to incorporate more context from the business I guess so the data that lands in the 3rd party tools is actually solid.

Once you have customers and a good network of integrations with a large number of tools, I suspect it's easier to just buy that company than build it all yourself?

This is exactly right. We even went so far as to build a proof of concept internally, and the technical challenges are just very different. The simplest way to explain it is that Fivetran connects a skinny pipe (APIs) to a fat pipe (databases) while Census connects a fat pipe to a skinny pipe.

The data is only as solid as you make it to be. Ultimately reverse ETL is just a technology (basically from SQL to APIs). The quality/correctness of data is someone else's headache. I've been there and done that, and reverse ETL is a feature-product with huge churn. See how Hightouch pivoted hard from that into CDP.