Is encrypt-and-MAC secure for AES-CTR + UMAC?
Encrypt-and-MAC does not preserve confidentiality because two encryptions of the same message will have the same mac tag, revealing the fact that the messages are equal.

Ssh includes a counter in the data the mac tag is computed over, so it never computes the mac tag for the same message twice (even if identical data is transmitted twice), so this particular weakness is not an issue.

This is not an endorsement of encrypt-and-MAC, however. Do not use it.

(If you have a strange MAC, encrypt-and-MAC can leak even more than equality, but few reasonable MACs are strange.)