As a dm, I of course reserve the right to change any rule that I think is a problem, but if I'm making the change mid-session I will always rule in favor of the player. So in the original example, I would let the feat work.
In practice, I just rarely go against RAW although I don't always agree with everyone about what RAW means.
On the other side, as a player - I do try to think about these things ahead of time. I'd like to think I would have asked the dm if I could possibly trip/slow/halt such a massive creature before choosing my action, since I presume my character would be able to see that "this thing is way to big for me to push around." Ideally I'd ask when I took the feat, and hopefully I'd ask before using the reaction.
I do ask about the interaction between booming blade and Warcaster as soon as I have access to the combo. I find RAW finicky but fair once you clear up some RAW vs RAI, although I'd be inclined to read "willing movement" as the same as Opportunity Attacks rather than relating to the creature's mental state. It ain't psychic damage after all. But if we can hash it out beforehand that's easily the best time to do so.
I would, however, be pissed if the ruling was given after the *re*roll.