Hi,
I agree ModernBill is a great application, unfortunately when we used it we encountered lots of issues and the support was terrible, for this reason we had to cancel our membership and have custom developed our own management system, we have never looked back.
Personally, I would never recommend ModernBill, but I know lots of people like the application and there are a number of hosts who use it

shame about the poor support and various limitations, but it’s better than nothing.
The ClientExec & PerlDesk management tool does look impressive, but I have never used it so can’t actually say.