There are several concerns here:
First, regardless of how slow it may be going, a moving coaster train is VERY dangerous. Therefore, puting a live actor in that situation would cost a lot insurance wise in addition to the breaks, sick days, etc.
Second, control of the Yeti might leave WDI. Right now, all live actors are under Entertainment (not WDI or Attractions). Since the Yeti would not be a "host," he would likely be a character. This adds onto the demands ad would make fusing the two together difficult. This might not be the case, but it is possible.
Finally, the AA will likely be more realistic due to the size than would a live actor. A costume could work, but the mechanics of these robots are able to create movement that doesn't look so "human."
Just a few thoughts I had.
Also, back to another post that I don't think got mentioned, the HM used to have a live suit of armor (where the one shifting his weight is in WDW's). The "armor" had a button to stop the ride, but several situations occurred. (1) the "armor" would touch guests from time to time (a big no no), (2) guests would attack the armor, and (3) it became a somewhat unwanted role due to the concerns and safety issues. Therefore, it was let go fairly early on.