[hinkley]

You can't pronounce authn and authz very well, but to be perfectly honest I'm not sure if that falls under the 'pro' or 'con' column.

[fragmede]

I think it's a pro. in saying auth-enn and auth-zee (zed), it's clear which of the two you're talking about.

[dietr1ch]

To me they look like the kind of abbreviations I'd only do when writing. I just say authentication or authorisation when reading them (out loud or in my own mind)