Using a transceiver IC instead of a ready made RF module means that you have to concern yourself with more details of the RF design, e.g. impedance matching or regulation compliance and also need to implement more protocol layers in your code. Even if you utilize reference designs and vendor software libraries, consider much more development effort.
There are many sub-1GHz transceiver chips on the market, e.g. CC1101/CC110L from TI, or several devices from HopeRF.