I don't know if people are necessarily down on Horni (I'm not) but perhaps a little disappointed in the lack of progress in the passing game overall and somewhat in the progress to his game. He's still not able to deal with pressure. In part due to his physical limitations, and to some degree due to his receiving corp.
The pick six last night was a good example. I suspect, that play had a certain timing in practice. Horni had his TE wide open and could have hit him, but Michigan disrupted his "timing" of the play somewhat and forced him to make the decision and throw quicker that how it was practiced. He's just not able to do that, and the result was a bad throw and a pick-6.
If I am an opposing defensive coordinator, the blueprint is there and has been for a couple of seasons on how to defeat the Badger offense. Commit heavily to "controlling" the running game. Force UW into passing situations. Then, bring pressure. Make AH uncomfortable and disrupt the timing by making him make throws quicker, and make him move. He's just not very effective in those situations.
That doesn't make him a bad QB and I'm personally proud to have him represent the team and university. He's a class kid and I think he should continue to start. At the same time it's reasonable to acknowledge his weaknesses. Not every team can do what I outlined above, and at those times he can destroy teams that commit to stopping the run if they can't bring pressure consistently. The tough part for UW is that the teams they need to get by - OSU, UM, and PSU typically can do that most years.