My very first startup (early 2000's) was a SaaS billing platform. It was acquired long ago, but I've yet to work on any system that approached that level of complexity. Ignoring all of the challenges of taxes and such, things rapidly scale in complexity as soon as one needs some level of flexibility.
For simple cases where entitlements are limited and the billing is generic, I think it's fine to built it yourself as long as you're aware of the time and cost of doing so vs. buying it. As others have said, time spent on building billing is time NOT spent on building products.
For simple cases where entitlements are limited and the billing is generic, I think it's fine to built it yourself as long as you're aware of the time and cost of doing so vs. buying it. As others have said, time spent on building billing is time NOT spent on building products.