q) Is it a warmachine or a chariot?
a1) It is a warmachine in all respects except moving (chariot), shooting (a little), magic (affected only by spells with S), combat (impact hits/ auto-hit), pursuit (it can't), special rules (like unbreakable, large target aside). So it's deployed at same time as warmachines, does not give VPs for being under half wounds, etc.
a2) It is a chariot for the sake of movement. Which means, unless charging, it can move and end up facing in any direction. This would suggest that for movement alone, it has no rear/front/flank. Using movement for justification of grinding positions would make no sense under RAW. Using charging rules would alter this somewhat, but we are not discussing if the Tank is charging as there's little way that it could charge and be hit in the rear/flank in the same turn.
I am playing devils advocate here so please bear this in mind when answering.