The double-entry problem, in the ordinary sense
A standalone khata app sits next to your billing rather than inside it. You write the bill for the customer and then type the amount into the app. That works while you are careful and stops working on the day the counter is busy — and once the two records differ, neither can be trusted without checking the other. In MaterialBill the bill is the ledger entry. There is no second action, so there is nothing to skip.
What a khata should record beyond the amount
A balance settles nothing on its own. What ends an argument is the detail: which bill, on what date, what material, delivered to which site, in which vehicle, received by whom. That is what a ledger entry generated from an invoice and a delivery carries. A number typed into an app carries none of it, which is exactly the situation where a contractor says a load never arrived and you have an amount but no document.
Where a khata app is genuinely enough
If your business does not raise bills at all — a tea stall, a small provision counter, a service where the transaction is just an amount — then a khata app is the right size of tool and a billing system is overkill. Be honest about which you are. Plenty of shops do not need GST invoices, stock or challans, and for them the simpler product wins.
What you gain by moving
Proper GST invoices with HSN codes, stock that moves as you bill, delivery records with vehicle numbers, statements you can send monthly, and a portal the customer can log into. What you keep is the thing you came for: a khata that is always current, ranked by how old each balance is, with a WhatsApp reminder that carries the party's own ledger.