I can only speak for what we've experienced first hand, in our prev. company (a $5B B2B fintech) we spent a lot of time around billing x invoicing in pdf :
- When billing is complex (lots of fees, different billing cycles, usage based consumption) - And you need to have a clear "invoice presentation" to avoid triggering questions (that is maintained while you iterate on pricing, launch new products or countries) the "template isn't really a solved problem", and our eng. team did rebuild the system in-house. We would have avoided it if it could have been possible and looked at every possible option, in-depth.
We've seen similar needs for marketplaces or vertical saas as well.
However thanks for the feedback, we could have done a better job at explaining why it can be a pain in specific use cases! And I was the 1st person to say, when I first discover the painpoint "there must be an existing solution for that, please don't build anything in-house".