No, I've seen the Radiants advance slow and carefully, even standing still like 10 meters from my front for a few seconds (before rushing in, killing everything).
But I've also seen the loose cluster charges you mentioned.
I think it's the standard bearer that makes them group together.
Anywa, you can deal with them. At first, I was lost. Radiants smashed my Imperial armies without difficulties, the horror. But then I started positioning my troops on steep terrain, packed them really tight together and added cannoneers.
It works, as long as the Radiants aren't larger in numbers. Whenever they reach your front in significant numbers, it's game over.