So, when you want to offer criticism on an in-progress piece of writing, constructive is the way to go.
That means that your end goal is the construction of a solid end product. You don’t get solid end products if you nuke the structure from orbit.
And if you’re particularly nasty, the author won’t be receptive to your feedback. They will feel attacked, and they will feel like what you’re saying is designed to destroy their work or make them angry. They won’t feel like you’re participating in their goal of a better product if the words you choose are about how bad it is now instead of how much better it could be if they apply your thoughts. If you talk like this, the author will likely feel that you’re primarily there to enjoy hurting them over something pretty personal.
And don’t ever imply that non-constructive feedback would be useful if only the author weren’t too sensitive. People can tell when you’re out for blood. Yes, there are authors who will snap and feel attacked if you say a single critical word, but most authors who would participate in seeking out feedback actually want to use your commentary to make their work better. Be a responsible critic and give them something they can use.