Remembered a similar topic I have read a few months ago where the OP asked a question that might help you :
I've noticed that mods have recently started locking such threads, or even threads with the potential of becoming one. It'd be interesting to see if there are any guidelines mods follow for locking such threads nevertheless.
And I think (he never really quoted the OP) this is what hilarious replied :
Probably depends on the mod, but I usually trash utter shit threads with little to no value, whereas ones that have just degenerated into spam that may have some half decent posts I usually lock. Sometimes a thread can start of decent but shitposters don't actually read the op and just the title and it quickly turns into a spam mega thread which will never recover.
It will still be subjective per mod who is viewing the megathread and it will always boil down to their own opinion as there are no guidelines for megathreads that are worth locking/reporting based on how I understand hilarious' reply. So you are basically alway putting some risk that your report gets denied when you are reporting a megathread.