Quick set of rules.
1. Can't be a specific companion or regeneration. Unless the reason is due to an unrealistic action taken by the character.

1. No answers relating to the inconsistency of lore/world building. Unless it's counter intuitive to the core of what makes that lore interesting/good.

2. No political jabs in any direction. While episodes "can" go overboard in the messaging, your flaw shouldn't boil down to differences in political views. Unless, the flaw relates to a character doing something awful that they definitely wouldn't do in any other episode/story.

3. No flaws should relate to behind the scenes drama OR disagreements between production staff and the BBC.

4. Finally, avoid mentioning the budget for the episode/story, this is a flaw with most of the show in general.