It may be important to notice whether you are meaning reliability or accuracy, or may be, availability.
The reliability should be regarded as the probability to fail on demand, and only associated to safety instrumented functions, performed by safety integrity systems, for risk reduction purposes. Only a careful analysis should determine on the convenience of using a UFM in risk reduction issues.
Accuracy will depend on the UFM technology (intrusive Vs. non-intrusive, single path Vs. multiple path, etc) and the process itself (dry gas, wet gas, biphasic streams, etc.), not to mention installation mistakes.
Finally, availability…compared to what, an orifice plate? In both measuring methods we will have instruments (temperature / pressure transmitters) associated, as well as flow computers - the difference is mainly the sensors. Obviously a steel plate is more robust than piezoelectric sensors (though you may get the impulse lines clogged). The information and diagnostics will be different too!

An ultrasonic flowmeter (non-intrusive Doppler flow meters) is a volumetric flow meter which requires particulates or bubbles in the flow. Ultrasonic flowmeters are ideal for wastewater applications or any dirty liquid which is conductive or water based. Ultrasonics flowmeters will generally not work with distilled water or drinking water. Aerations would be required in the clean liquid applications. Ultrasonic flowmeters are also ideal for applications where low pressure drop, chemical compatibility, and low maintenance are required.