"Advantages of whatever THAT happens always outweigh the costs"
Grammarly underlines THAT as red to leave it out. I know that when I say "whatever happens, I will be there for you" there is no need for "that. But when I use it after "of" I feel like a "that" is called for.
If you agree or not could you please ascertain why or when one uses "that" with "whatever, whenever,etc" if ever..