Synthetic small caps does not handle context sensitive case mapping rules as in Unicode 8.0 D138 Table 3-14. Context Specification for Casing because it we don't have a streaming case mapping API in ICU. Such an API would be needed to efficiently determine whether an individual codepoint in a text run would contribute either one or more than one codepoints to its case mapped counterpart.

The feature request is filed for ICU in 
http://bugs.icu-project.org/trac/ticket/12410