Secure rot* variants require UTF-8 and mappings that shift characters between {1,2,3,4}-byte encoded-character-sizes. That varies the message length, which prevents any message-length or traffic analysis.

The Snowden leaks revealed that the NSA is flummoxed on how to tackle variable character lengths. However, they've cracked rot26 using custom ASIC supercomputers, so it should be considered insecure even though it's twice as good as rot13.