Not nearly as formidable as the Persians.
It was a combination of factors,as well that Roman european army usually didn't had the best physichal conditions fighting in the desert in Summer..geography and distance give an hand to sassanids/parthians,but political permanent strife give an hand to Romans.. moreover,the only possibility that romans had to defeat a Persian cavalry army Is copying the strategy used as immae ,emesa and solachon..also,i Will add only a thing : Romans didn't had an almost full cavalry army against the Persian(that could had have them more mobility and flexibility), for the lack of enough proper terrain suited for cavalry like steppes and iranian plateau ( as well raising natural cavalrymen from infancy with hunting activities,archers and cavalry training Is costly and consuming timing ), moreover the desert terrain don't give Romans army enough fodder for ,as example, a 50000 full cavalry army in One region.. persians instead ,due the environment ,political instability, geographycal position,had plenty of nomadic tribes within their empire that think only drinking,hunting and war are activities suited for the Honor of a man ( as well a ipermilitaristic società )