At what scale do cash flow and ops become real problems ?

For those who have scaled a store:

• At what monthly revenue or order volume did operations start to feel difficult to control?
• When did cash flow management become more complex than simple profit vs. expense tracking?
• What became the biggest constraint first:
  • Inventory
  • Marketing spend
  • Fulfillment
  • Or team/process overhead?

Looking to understand where the real breaking point appears during rapid growth from people who have actually experienced it.