i think that you should change the moving way of the UFO because since it is a 3D type of game, it should move with the camera or some sort of stuff . because when i play, i row the cam to the front of the UFO and press forward and i goes in the opposite direction. and if this was your first unity game, i think that it went well. but next time you create a new ufo games, i think you should gave it a purpose, like steal cows or laser attack like xenocidical idea, i like that too.
so why should we grow the tree any way? i have a few HP left, monsters suround me but i think: no bother, the tree only have 3% more to go"...i let the monsters hit me, tje tree finished and i was like yay yay then the tree turn out to be a monster and i was like wtf what have i saccrifice for? then i'm dead >.<
i think that invisible zombie are is cool but you should make them a bit more noticeable like when it hit some one, it blink, some thing like that
thank you.