I noticed it several times now. If the opponent has only one unit on the board you can not activate tainted ale. There is no such limitation in the description though.
So how does your Tainted Ale work? It clearly states: Damage a unit by 2. However, there also seems to be a reach limit because if I put it on ranged row I cannot target the opponent's unit on the opposite ranged row. Got me in quite a pickle in Arena...