It is TOO simple. It seems to not take cues from other invoicing software, just does its own thing, and it makes things not user friendly. Creating an invoice is not a simple task, and you can't just duplicate an invoice to send to a client again. They hide things under pay walls or complicated features that should be easier to use.
What used to be free, they kept going down and down, now you can only have 1 client on free mode. Why was it ever free? Why tease people just to completely remove everything?
I cannot reiterate how annoying it is to create invoices. Now that it is paid, there is no reason to use this. Go back to Freshbooks Review collected by and hosted on G2.com.