Just to give you an idea how complicated default template is becoming.
Consider this topic: BTW (TAX) Subtotals on Invoices - #13 by Hennie
If you are from Netherlands and using two BTW (VAT) rates on invoice at the same time, according to Dutch legislation you need to show BTW-exclusive amount for each tax component. Again, this is some obscure rule which applies to one country only and will be incorporated in default template.
As for Liquid variables, default template is also using only variables listed on this page: Manager Cloud
It’s just that default template is written in C#, not in Liquid.