The issue with the Non-PIS NRO account is the burden of TDS calculations which refrains the brokers from offering to trade using NRO account without PIS.
The TDS calculation is a huge operational overhead for the brokers and is difficult to handle with the existing capacity. In the case of PIS accounts, the TDS calculations are handled by the PIS bank and the brokers are not required to do anything at their end.
However, in the case of the Non-PIS NRO account, brokers will have to get involved in end-to-end TDS calculations before crediting the proceeds to the NRO account. Thus many brokers still insist to trade using NRO PIS account despite waiver from RBI to trade without PIS for NRO mode of transactions.
Currently, Prostocks and Tradeplus offer to trade using NRO without PIS.