Many of the tactics puzzles punish you for even slightly suboptimal, yet still trivially winning moves.
http://en.lichess.org/training/24964
It is clear to me that the final move of capturing with the queen forces an easy rook up endgame (counted as a fail), and not so clear why the knight capture is "better," or even that it is at all. I think most humans would favor trading down to a very clean, trivial win. This is only the most recent example among many and it is putting me off your tactics trainer.
http://en.lichess.org/training/24964
It is clear to me that the final move of capturing with the queen forces an easy rook up endgame (counted as a fail), and not so clear why the knight capture is "better," or even that it is at all. I think most humans would favor trading down to a very clean, trivial win. This is only the most recent example among many and it is putting me off your tactics trainer.