True horizontal mirroring, flipping a letter left-to-right like a reflection, as opposed to upside-down text which flips top-to-bottom, is much harder in Unicode because very few characters have a genuine mirror-image look-alike. This tool reverses the order of your characters and then substitutes each one with the closest horizontally-flipped Unicode symbol it can find (for example, 'b' becomes 'd', and 's' becomes a reversed 'z'-like glyph); letters without a good mirrored counterpart are left as their normal selves since a wrong substitute would be more confusing than none. Because of that limited coverage, mirror text reads best on short words and names, long sentences will contain a mix of flipped and un-flipped letters, which is an inherent limitation of the trick rather than a bug.
Unicode doesn't have a true horizontal mirror-image for every letter, so characters without a decent mirrored look-alike (like m or w) are left unchanged rather than replaced with something misleading.
Upside-down text flips characters vertically (rotates 180°) using look-alike glyphs, while mirror text flips the character order and substitutes horizontally reflected look-alikes, the two tricks use different Unicode substitutions and look noticeably different.
It works best on short words or names, with limited Unicode coverage for true mirror-image letters, long sentences end up as an uneven mix of flipped and unflipped characters.