I used to be one and my RT was ****e. And its also a capacity thing until your patter is subconsious the demonstating, pattering, keeping you SA of local traffic, listening to the RT and replying when talked to is extremely high work load.
Which is why you hear quite often "I have called you x times" alot of controllers presume the instructor has turned the radio volume down when actually they are just maxed out. 50 to 100 hours and you have a subconsious ear out and can respond without having to think to much.
I also fly with and train FO's that are 200hours or less and know exactly what they are like on the RT. A 25 hour FIC doesn't change things.
Also alot of the instructors have done the majority of their training abroad so might only have the IR (15hours) and the FI (25 hours) courses using CAP413 in UK airspace.
If they did there PPL and hour building in the UK they are significantly better but still when you add on top teaching, monitoring etc it all falls over because basically your SA has suffered and the RT call comes as a suprise. Instead with a bit of experence you have in the back of your head they are about to pass me traffic info on that cherokee thats just booked in thats coming in from the North 15 miles out so don't have to look for him for another 5 mins but if I turn westerly I will keep out the way all while pattering straight and level part 1. So you know when you get the traffic info you know what the responce is going to be.