This issue has been bothering me since I started using Figma a few years ago:
The rendered px units in Figma are about 8% smaller than they are in Chromium-based browsers.
In other words, when developers copy px values from Figma and implement them on the web, the rendered results are around 10% larger than the design.
This is a really serious issue, especially when selecting very small font sizes - for example, there is a considerable difference when a font is rendered at 10px, 11px and 12px, and designers are very careful when selecting small font sizes.
And just in general, you design for a specific experience, especially when creating fluid web page designs - a 10% difference is a huge perceived difference on a large page, and it just gives the design a different feel and experience versus what you actually designed.