Robots may be cheaper than crews, but they also have several downbacks (at least IMHO):
First and foremost, you must remember that true artificial intelligence is beyond TL15 (and I guess that's also true in MgT), so any problem not foreseen by the programer will be unsolved, or have only the department chieff to solve it, being the only true intelligence with some knowledge in the area that can solve it.
Also, I guess the Pilot must be human, only if because people will want someone (not someting) to blame should any serious problem arise. You can not sue a robot before the courts (at least according Canon, AFAIK).
Again my guessing, but I foresee stewards also as people, not robots, as customers would be more comfortable. Any pesky passenger (and I guess there will not be rare) asking for something unusual will always get a better answer from a person that can understand him (or arge him) than from a robot whose answer will just be "that's not in the menu". You use to want a little empathy form your bartender, aside from your drinks.
I expect similar problems with a full robotic medical crew (while robotic aid may help).
For engineering, aside from what I said above about unexpected problems, as the engineering robots (or repair drones) are depicted in MgT, they are too small should any strength be needed (e.g. to move a heavy/large part).
Also, in case of pirate attack, hickjack attempt, etc., the crew would be too small to repel it (and I guess security robots would be seen as unneded, as you cannot double posts with robots, they being specifically built for their mission).