trenchant
Very sharp and clear, cutting right to the main point.
When you describe someone's criticism as trenchant, you mean it's sharp, clear, and gets right to the heart of the problem. A trenchant remark cuts through confusion and excuses to expose what's really going on. The word comes from an Old French term meaning “to cut,” like a sharp blade slicing cleanly through something.
A teacher might make a trenchant observation about why a student keeps making the same mistake. A trenchant comment about a school policy points out exactly what's wrong with it, in a way that's hard to argue with. When a book reviewer writes a trenchant critique, they identify the book's real weaknesses precisely and clearly.
Notice that trenchant doesn't just mean harsh or mean. It means accurately sharp. A trenchant remark is one that's both cutting and true. If your friend makes a trenchant point about why your plan won't work, they're being honest and helpful, even if it stings a little.
Trenchant analysis, trenchant wit, and trenchant commentary all share this quality: they're sharp, clear, and effective.