Targetmasters make the most sense for their partners in vehicle mode, as mentioned before, having a separate gunner from the driver/pilot.
And, yeah, the biggest issues with humans or Nebulans being Head/Target/Power-masters is 1) the surgical modifications necessary to convert would be extremely extensive, and 2) you now have creatures with pretty finite lives bonded to machines with not-quite-infinite lifespans; eventually this would have to cause problems that maybe can't be addressed in the usual Transformers media.
And, yeah, the biggest issues with humans or Nebulans being Head/Target/Power-masters is 1) the surgical modifications necessary to convert would be extremely extensive, and 2) you now have creatures with pretty finite lives bonded to machines with not-quite-infinite lifespans; eventually this would have to cause problems that maybe can't be addressed in the usual Transformers media.