Over the years, I've tried to use Unicode for math symbols on various web pages and tend to revert back to GIFs or LaTeX-generating tools due to problems with symbols missing from the font used by this or that browser/OS combination, or even incorrect symbols in some cases.
IMO the biggest problem with Unicode is the lack of a public domain reference font. Instead, it is a mishmash of proprietary fonts each of which only partly implements the spec. Even the Unicode spec itself uses proprietary fonts from various sources and thus cannot be freely reproduced (it says so right in the spec), a terrible idea for a supposed "standard".
I'd love to see a plain, unadorned public-domain reference font that incorporates all defined characters - indeed, it would seem to me to be the responsibility of the Unicode Standard committee to provide such a font. Then others can use it as a basis for their own fancy proprietary font variations, and I would have a reliable font I could revert to when necessary.