What you can do rather reliably is to compare the following:

• GetFiles() -> GetLoader(). If you get same loader instance, you can then compare reference keys;
• GetIndex() have to match;

If you loader is always IDWriteLocalFontFileLoader, you can then get a path directly, and compare that.

Equals() will consider simulations too, meaning faces using same {fontfile, index} could still be unequal.