At some point, things were working, so I'm not sure what changed recently. When adding this pod, I'm receiving the final error from cocoapods:
[!] The 'Pods-MOJO' target has transitive dependencies that include statically linked binaries: (Analytics)
This seems to relate to using use_frameworks! in cocoapods. Removing that causes another error the #import <Segment/SEGAnalytics.h> can't be found.