Swapping 'unmatched' for 'unprecedented' can absolutely work in fiction dialogue, but it's a subtle swap that changes the flavor of the line more than the core meaning. To me, 'unmatched' usually carries an immediate, emotive punch — it's shorthand for 'there's no one else like this' or 'this beats everything else.' 'Unprecedented' slides toward the historical or clinical side: it implies 'never seen before' and often sounds like a verdict rather than a feeling.
If I'm writing a scene where a character is bragging or someone is marveling in the moment, 'unmatched' keeps the heat and intimacy. For example, "Her skill is unmatched" feels direct and personal. But if the
speaker is making a broader claim, summarizing an event, or coming from a more formal register — a commander, a historian, or a scientist — then "unprecedented" fits better: "This level of destruction is unprecedented." You also have to think of rhythm and syllable count; 'unprecedented' is five syllables and can weigh down quick, punchy dialogue unless the cadence supports it.
I like sprinkling alternatives based on voice: 'peerless,' 'unequaled,' 'without parallel,' or even conversational turns like 'like nothing I've ever seen' or 'no one comes close.' In short, the swap is possible, but only if you consider who’s speaking, how they speak, and what emotional register you want — and I usually pick whichever word preserves the character’s breath and rhythm. Honestly, I tend to reach for the word that sounds like them, and that's where the magic happens.