That Monet could be killed By Caesar Clown by stabbing its heart looks a bit awkward to me.
I mean, when Monet was cut by Zoro she didn't look to be prepared to properly activate its logia power to receive the cut and she still was able to survive and regenerate, so it's safe to assume that he can instinctively react, or that even if she got damage on her brain, she was still able to avoid it by converting into it's logia element, and it's sure worse to have your brain splitted in two than having your heart stabbed.
Maybe Law's powers made her unable to use her logia powers, but it's clear that any cut part that is under Law's power still can react to its owners will as shown with Kinemon's legs.
Then maybe it's that Logia users cannot convert their heart to their logia element, so that would be a weak point they have.
I'm asking about the possibility of logia users not being able to convert their heart into their element, so it would be another weak point they would have apart from being attacked with haki. I mean, they could also be killed, for example, by stabbing them with a sword where their heart would be, no need of haki to do that. I'm giving those facts as a support for this possibility, so I'd like your opinion if that's really a fact, a possibility, or there's something that makes the possibility I'm telling impossible.