WIth a simple string of letters ABCDEFGHIJKLMNOPQRSTUVWXYZ in math mode in either LaTeX (via unicode-math) or Word we can see that STIX Math is not properly kerned, unlike the commercial font and the first with OpenType Math table, that is, Cambria Math.
Anonymous
2012-12-30
Comparison of kerning of STIX Math vs Cambria Math
stipub
2014-06-11
This issue is targeted to be fixed in version 2.0.0 which is currently under development.