I suppose that means writes slow down with the number of dependencies/ dependent caches? How does one estimate the added cost of that write?
We have a limit - you can only "tag" up to 1k items within one query. If you have more items in there, we can't purge all of them.
Due to the nature of the broadcast protocol of Fastlys purging implementation, I assume that it scales quite well - they have really big customers using this for a few years already.
We're using Fastly ourselves and I was not aware up until today (and in fact I'm taking your word for it, since I can't find it in the docs) that fastly provides consistency guarantees for purges.