- Font rendering is very inconsistent across platforms, so your icons can look pretty bad (e.g. on Windows)

- You're more likely to hit aligment/layout issues because you're using a tool designed for text to render pictures

- The character codes used are textual information but have no semantic meaning (this can be addressed by using ligatures or characters in the private unicode range)

- Adding more icons requires regenerating the font

- Using a subset of icons requires regenerating the font

- Font authoring is non-trivial and requires much more specialized tools and knowledge when compared to SVG

- You need to ship the font in various formats (TTF, OTF, WOFF, WOFF2)

- It's not easy to animate icons, or include colors or transparency

- By now SVG support is sufficiently widespread, so the original rationale for using font icons no longer applies

None of these, except maybe the rendering quality, are deal breakers. Workarounds and counterarguments do exist. And SVG does have some disadvantages too (e.g. parsing overhead, filesize, memory consumption)

I personally side with using SVGs currently. As technology progresses, I might definitely reassess my view.