What things does UHD do?

Hello everyone here,
I find if I do not use UHD, for example, when I use gnuradio-3.3.0 to
transmit files, the “ok=flase” when I set the bitrate greater than 600K
less than 300K. However, when I install UHD and use gnuradio-3.5.0, I
I can use the bitrate from 1M to 5M, why the uhd can lead to the
when I use the same modulation method?

I get bitrate from the URL:
Bitrate(symbols per second)=convert_rate/inter/sample_per_symbol
The convert_rate is decided by the DACs, the inter and
is set by gnuradio, does them lead to the difference?
Any guidance will be appreciated