When new fonts are released, they always include what they tried to improve: readability, comprehension, etc. Just once I'd like to know what they sacrificed.

In this case they sacrificed a feeling of professionalism. Helvetica is "serious" and used by real publications. Kermit would probably not be used by a major publication (like NYT or WaPo) because people wouldn't take them seriously even if it's easier to read.

Variable font width, height, and kerning is more difficult and slower to read. It's fine if you're reading a short childrens book at out loud, but if you're reading an entire novel silently formatted like that, it would become exhausting quickly.

In this case its subpixel rendering on low-dpi displays.