I was getting failed builds on windows trying to build with expo for android. I don't have the error messaging now but the fix i put into place was as follows
in node_modules/@unomed/react-native-matrix-sdk/android/CMakeLists.txt
# Convert Windows backslashes to forward slashes for regex compatibility
string(REPLACE "\\" "/" UNIFFI_BINDGEN_PATH ${UNIFFI_BINDGEN_PATH})
Putting it here in case it helps anyone else!