It's pretty crazy that this thread is 10 years old. I don't think I've seen modernbill in equally as long.

Modernbill, for all it's strengths, also had some weaknesses. For example, whenever you loaded ANY page in the whole billing system, it loaded an HTML dropdown of ALL of your clients. That's fine for a couple hundred clients, but it doesn't scale. 100k, 200k clients and suddenly page loads were frustratingly long.