I can see the value of having an OSS solution as an API layer for any start-up that has complex billing needs.

A lot of start-ups make the "mistake" [1] of locking themselves into fundamental business services at the start: whether that's a payment provider, finance or CRM system. In my experience it's fine for a while, but you really feel the problems as you start to scale the business. Everything creaks and groans as you grow, and then you have to go back and fix the underlying business processes. Issues in some of these areas are actual company killers. Billing is a classic. Lots of companies fail due to running out of cash, and poor management of subscriptions and billing is often the underlying issue. There's a lot to be said for not getting tied in early on, and making sure the fundamental architecture is right!

[1] You can argue it's not a "mistake" as the primary goal is to relentlessly focus on shipping product that customers want.

There are small things - e.g. you can't do weekly / fortnightly billing.

Once I discovered that I stopped investigating BT for our primary subscription engine. We just hold the card tokens and use BT to charge the card.

It's not ideal, but it does give us more flexibility.

Not substantially. You'll need to do minimal, common-sense things on your servers (firewalls, IDS, etc) and certify that you're not storing anything interesting.