From this www-font posting by Adam Twardoch
Obviously, SVG Fonts have some good and interesting concepts. One of
their advantages is that they can – at least in theory – freely
combine all aspects of SVG: multi-colored, multi-layered vector
graphics, and bitmaps.
However, SVG Fonts also have some serious drawbacks: while the glyph
definition using SVG is a great concept, all the other aspects of SVG
Fonts that make them work as a font, especially the character mapping,
access to alternate glyphs, and the layout behavior, are somewhat
under-defined and hard to implement. Therefore, it’s rather unlikely
that at any time, all OS and application vendors will agree on a good,
full implementation of SVG Fonts.
Therefore, I’d like to suggest a different path: place an SVG Font as a
table inside of an OpenType font*, and combine the power of both formats.