Castform is typically a fairly poor choice of a Pokemon in any tier; NU is truly no exception. You can pretty much find this out by reading the on-site analysis for Castform found here. Its only set is called "Everybody Look At Me I'm Using Castform."
I'd be all for a legitimate discussion on the merits of Castform if I felt that there were any. Typically when posting a thread to discuss a Pokemon, it's good to bring up some discussion points in order to lead users as to what exactly you would like to discuss. In the case of Pokemon that seem mediocre that you have had success with, it helps to make an argument justifying its usage on a team.
Castform is generally an outclassed weather abuser. It doesn't have any over-the-top stats, it has a poor offensive typing, though it has good coverage. Base 70 stats across the board makes it a poor choice of a Pokemon in NU, even PU.
I don't have much to say other than that, but if you're using Castform in a competitive team, you're definitely doing it wrong.