What B2B SaaS infrastructure do you keep rebuilding?

I've noticed that a lot of B2B SaaS projects end up rebuilding the same infrastructure:

• Authentication

• Organizations and multi-tenancy

• RBAC

• Invitations

• API keys

• Usage limits

• Stripe billing

• Webhooks

• Audit logs

• RLS

I'm curious what other builders have found themselves rebuilding repeatedly.

What's the piece of B2B SaaS infrastructure you least enjoy implementing from scratch?

And if you've shipped a multi-tenant SaaS, what was the thing you discovered too late?