I've been told apparently the CE/CMO can NEVER deploy. It is written in ML that the CMP can never deploy off the ship, but nothing in SOP or ML as of this post says CE/CMO can not deploy.
There are certain scenarios where I feel its valid for them deploy, such as secured groundside surgery with a full medbay staff for CMO, or a lack of/ dead engineer so CE deploys to secure FOB.
That said, my main gripe is that this is apparently a HIDDEN RULE that you can't find in the current SOP or ML. Regardless of debate over whether there are valid scenarios for the CO to authorize the CE/CMO to deploy, if this is a rule that players are to follow, it should be written out where players can read it and should be made obvious to them.